Short Answer
Part of the issue is that upgrading the power transmission infrastructure to accommodate large numbers of new renewable power sources takes considerable time and money.
Long Answer
In the short term, one of the issues is that renewable power generation facilities are not in the same geographic location as fossil fuel power plants. Also, it isn't just that there is a new geographic location. Generically, renewable energy power generation facilities operate at more locations, with less power generated at each location, than a fossil fuel power plant. So, there are far more power sources that need to be connected to the grid with current technology renewable energy sources than there are with previous technology fossil fuel and nuclear power plants.
And, while renewable energy power generation has made incredible technological improvements in the last decade or two, electrical power transmission technology peaked at something close to the theoretical maximum efficiency and cost level (barring the development of affordable room temperature superconductors or a dramatic drop in the price of gold) in the 1950s or so.
This means that to include new renewable sources of energy into the power grid one has to build high voltage transmission lines and transformer stations, i.e. the literal "grid", to get the power from where it is produced (perhaps a farmer's field with a wind turbine, or an airport parking lot with solar panels over it, or an off-shore wind turbine) to the rest of the existing power grid.
In the case of distributed sources of renewable energy, like rooftop solar panels on homes, it means upgrading the electrical boxes and meters for each home that is producing power from a one directional system to a bidirectional system. Each building's meter change has to be handled as a distinct transaction and costs someone something on the order of hundreds of U.S. dollars each for parts, labor, and administration of the program. Each installation may take a month or two to arrange even though the actual job may only take a few hours. And, the tens or hundreds of thousands of building owners involved each have to make the individual decision to invest in solar panels over many years. If there are surges in the desire to transition, furthermore, the supply of suitably skilled technicians may limit how quickly the work can be done.
While these infrastructure improvements aren't cost prohibitive, the cost of these infrastructure upgrades isn't negligible either. In the U.S. state of Colorado, for example, which has about 5 million people, it was projected to cost $15,000 million to build new long distance transmission lines to serve new renewable power sources in order to reduced carbon emissions by 87% by 2030 (as of 2021). This is about $3,000 per person in the state over nine years. It is mostly a one time expenditure, with a useful life of several decades, but it has to be done immediately before the grid can benefit from the new renewable power sources.
Equally important, building and installing new electrical meters, transformer stations, and high voltage power lines across long distances to new locations just takes time that is measured in multiple years or even decades.
Building high voltage power lines to off shore wind turbines is a major civil engineering feat of its own, although one that requires only proven technology and one that is understood well. And, as students of politics know, almost all major civil engineering projects end up behind schedule and over budget sooner or later, often dramatically so.
Building high voltage power lines over land requires the electrical power utility, in many cases, to obtain easements over land and to acquire small plots of land to seat the towers upon over hundreds or thousands of parcels of real estate (and sometimes local government approvals). This can take years before a single power line tower is built, unless you can use existing rights of way like rail and highway corridors. And, using existing rights of way comes at the cost of increasing the distance of power line connections, which increases the percentage of the power lost in transmission.
This would be an issue even if intermittency was not an issue and if renewable power generation infrastructure itself were much cheaper than it is now.
Electrical Engineering Footnote
At a fundamental physics/electrical engineering level, moving electricity from one place to another over wires, results in a certain percentage of the power transmitted being lost for each kilometer travelled. This is due to "resistance" in the electrical line which is the electrical equivalent of friction.
The percentage is mostly a function of the materials used in the wire and the voltage in the wire. Less power is lost in materials with high conductivity (e.g. gold conducts electricity better than copper which conducts electricity better than steel). And, less power is lost when voltages are high.
Electrical engineers work out the most cost effective combinations of materials, transmission line paths, and voltages to get electricity from the place where it is generated to an end user of it.
The electrical lines are also limited in the amount of power that they can carry, with high voltage transmission lines requiring bigger physical power cables than the low voltage transmission lines that connect to your home or business.
High voltage, long distance transmission lines are usually hung off tall transmission towers, to avoid coming into contact with vehicles, buildings, trees, and small hills. A typical high voltage transmission line tower takes about the same land area as a two car garage.
One transformer station increases the voltage of power from the power source going into the transmission line, so that it can lose less electricity en route to the consumer, while another transformer station near the consumer reduces the voltage as it comes out of the transmission line, after which it is carried on lower voltage transmission lines (which are generally already in place) the last mile or two to your home or business.
A transformer station will typically take about the same amount of space as one or two single family home lots, although the size can vary considerably depending upon how much power is handled at the transformer station. There is a considerable cost/engineering difficulty premium to making transformer stations smaller, as opposed to constructing them in the typical utility company fashion (they are quite simple in principle, a second year physics or electrical engineering student could design one).
Land acquisition footnote
To acquire line or put a power line over someone's property, the utility company typically has to:
prepare an exact map showing where the power lines, high voltage power line towers, and transformer stations are located that connects exactly to the existing grid and to the electrical engineering requirements of the project,
do title searches to determine who owns each affected parcel of land,
hire appraisers to determine the value of the affected land (which is a specialty form of appraisal practice),
make offers to buy or lease the land or easement (i.e. right to have a power line over land) from each owner voluntarily (sometimes with negotiation from mid to high level utility officials or lawyers), and
when owners don't agree, the utility company has to start a separate eminent domain lawsuit to condemn the land or easement in exchange for a court determined fair market value, for each parcel of land where the owner does not agree to a voluntary transaction or is non-responsive.
Once the owners of the property in question are located, it can take two to six months to litigate sufficiently to allow construction to begin (utility companies typically have a legal right to force a sale of land over the owner's objection), and it usually takes several additional months for each eminent domain lawsuit to be concluded.
Often the utility company will also have to seek a building permit from each locality in which the power line will be built, and/or will have to negotiate with local governments over condemnation or voluntary agreements related to local government owned land.