While one can imagine an ideal world in which the political landscape is dominated by a "pull" paradigm (voters actively go out to find the information on the candidates), in the real world it's dominated by the "push" paradigm (voters passively receive information given to them). If you're asking why we can't have the first instead of the second, well, that's not what's happened. The fact that we don't already have that shows that it's just against human nature. One can call it laziness, or rational ignorance, but whatever you call it, that's just not how humans naturally behave. A government website where candidates can put up their platform simply can't compete with daily bombardment of messages regarding current events. Even if some voters visit the site (and most won't), they're not going to come back to it day after to day to see commentary on the campaign as it unfolds. It's the same reason why companies run ads, rather than just putting up a website telling people how great their product is, and then sitting back and waiting for people to visit.
If you're suggesting that we force campaigns to be run that way, consider: What is campaigning? It's going around telling people why they should vote for you. In other words, it's speech. Which is protected by the constitution. There are some that argue that it's money, not speech, that is being regulated, but when you prohibit people from spending money on speech, you're regulating speech. Campaigning can be categorized into four main types:
Self-financing: A candidate uses their own money to fund their campaign.
Independent expenditures: Non-candidates use money to fund a campaign that is separate from the candidate. The candidate does not have any access or influence over the funds.
Media Coverage: Candidates can get exposure by getting the media (and this includes not only "establishment" outlets such as TV news and newspapers, but also social media) to give them attention.
Contributions to candidates: People give money to a candidate, and the candidate decides how to spend it.
The Supreme Court has found that the first three types are constitutionally protected . The restrictions on the fourth are allowed, but eliminating contributions to candidates would just leave self-financing, independent expenditures, and the media as the only allowable campaigning methods. The first obviously favors wealthy candidates, the second means that campaigns are not accountable to the candidates (they are legally required to not be accountable), and the third allows large media corporations to dominate elections, as well as rewarding divisive behavior (the best way to get air time is to say something controversial). Banning contributions to candidates doesn't take money out of politics, it just gives more of an advantage to those who can pay for their own campaigns, have proxies act on their behalf, and/or manipulate the media.
 Because this has involved striking down laws that prohibit speech based on how much money is spent on that speech, is it often characterized as the Supreme Court saying that money is speech. The Court has not said that money is speech, it has said that regulations on how much money can be spent on speech is regulation on speech, which is quite different. If there were a law that says that no one is allowed to spend more than $100 per year on firearms, that would clearly be a law regulating firearms, and acknowledging that fact would not be saying "money is guns".