The reason I think is they keep wanting planes to do more and more. It's not enough that it does what the previous one does, it has do to it all better. Then, it has to be able to do more as well. Then it has to be safer, then it has to be more survivable, then it has to be more lethal, then it has to more effective, and so on.
Well, that stuff isn't free. You have to pay for it.
Then Congress balks at building more. You can get 100 planes for $50 billion, or 50 planes for $35 billion...well, that saves us $15 billion! Uh, yeah, but now your unit cost has gone up.
Penny wise, pound foolish, every time.
The USAF bought the F-16 because they couldn't afford enough F-15s to do everything. We're buying the F-35 because of the F-22's price, but there are issues with the F-35. So the debate for more F-22s takes a different course...