Why won't the U.S. move more rapidly towards modernizing its army? By that, I mean why, for instance, the U.S. won't replace conventional fighter jets with attack drones and replace big and inefficient aircraft carriers with carriers specifically made to carry drones? Is there any reason they seem to be reluctant to rapidly move towards a more modernized army?
It's expensive to replace working (if old) machinery with new machinery. And the United States already has the most expensive military in the world.
There are times when drones are less effective than on-board pilots. In particular, what if the US were to go to war with someone with the capability to jam the drone control frequency? The drones would then be useless. Meanwhile live pilots could just proceed with their mission under the same circumstance. Or return to base. It would depend on the parameters of the mission.
US drones are currently designed to require human intervention to actually fire. The military is somewhat reluctant to allow drones to operate autonomously. There are concerns that they might be compromised.
Pilots expect to recognize the voices relaying orders. Automated systems do not. As such, in a real war, drones are subject to compromise in a way that pilots are not.
Why are drones becoming more prevalent at all then? Because the US is currently mostly fighting opponents with a large technology gap. And it is much safer to use drones for surveillance and fighting someone who can't compromise them. After all, it keeps the pilots from being at risk.
Since the end of the Cold War America has had no serious rivals. American technology, even though it is decades old, is still some of the most advanced military hardware in the world. Furthermore, the lifetime of this hardware is often extended by digital upgrades or material upgrades. This kind of upgrade does not require a hardware redesign, it just means specific components will be replaced or added.
The nations which American forces are most likely to tackle; Russia, China, North Korea, Iran, have large militaries whose hardware is technologically behind America's, and whatever quality exists does not have consistent quantity. Both Gulf wars went a long way to prove that American forces remain unparalleled globally, in part owing to the supremacy of western military technology.
This in turn increases the value of established hardware brands by becoming battle tested. The Abrams tank, or the Apache attack helicopter, demonstrably work very well. The value of future hardware is low while the value of present hardware is high.
Given all this, there is no justification for large orders of completely new things, like drone aircraft carriers. Successive projects have been cancelled, like the Comanche attack helicopter or XM8 rifle. As defence budgets have shrunk since the end of the Cold War funds have been allocated to maintaining current military-industrial infrastructure. It's more cost effective to spend money keeping a factory open than creating a new one to supply unproven technology.
American politicians have a preference for supporting corporations and thus jobs which already exist to supply proven goods. Even if the Pentagon keeps telling them to stop buying stuff it doesn't need. There isn't budget, need, or niche for massive modernisation programs. Present hardware fills all possible combat roles.
If anything, more hardware is being phased out than added. For example, we have gained a need for drones, but have lost a need for ground-attack planes and multiple rocket launch systems.
It would be a different story if the USA faced a technologically equivalent foe with equivalent ability to project power globally. Especially if present American hardware was inadequate or obsolete. Especially if there was a political need for more manufacturing jobs.
The US is rapidly modernising its armed forces, it's just that for defence contracts 'rapid' is not as fast as in the civil space.
Look at the development of the F35, they started the production in 1992 and it's only just starting to be used in the last year or two
To get a UAV with similar combat capabilities is going to take a long time, as the state of the art AI is just not that capable yet.
It's very difficult to tell what will be required for the future, will combat aircraft still be a threat if Laser weapon platforms becomes effective?
The introduction of a new weapon systems like this could make all current military hardware obsolete, in which case the US would lose its incumbent advantage and it would be a race to build the new weapons as happened when the UK developed the dreadnought triggering an arm race that is considered one of the causes of world war one.
TL;DR: The US is already introducing new weapons as fast as it needs to to keep itself the top military in the world