Its because of Preemptiveness (see
https://en.wikipedia.org/wiki/Preemptive_war).
So, the US has floating, mobile airports so that it can strike anywhere in the world to prevent a larger conflict from happening. "Anywhere" means anywhere in N. America, S. America, Africa, Middle East, Europe, and the Pacific -- that's a lot of area if you think about it.
Having a mobile airport can reduce transit time by days.
Also, there might be 10 carriers but they go on rotation -- like, 1/3rd are in dock while 2/3rds are out at sea. So, really, its like 6 or 7 carriers around the globe and 3 are being maintained (and out of duty). This is another reason why there are so many carriers -- there's a need for other carriers to cover for the ones being maintained.
They're not just floating there doing nothing either. For example, an aircraft carrier might be where a SEAL team begins their Operation. The team would then board a helicopter to go into, lets say, Somalia, for a hostage rescue.
If the carrier wasn't floating near the horn of Africa, then the US might not have been able to carry out the mission since, if no country within 1/2 fuel tank wanted to house the helicopter and team, the mission would have been impossible.
So, yeah, there's a strategic need for carriers.