There is
not an unlimited supply of potential prisoners. As wmil says, imprisonable crimes are generally those arising from poor impulse control and low intelligence. (If you want to learn more, Russell Barkley has some informative books on the connection between brain executive function, intelligence, and life outcomes.)
If we made wearing socks a three-strikes-eligible law, the same people would go to prison. Normal people would adapt, impulsive and dull people wouldn't. American criminal law is basically a test of certain neurological abilities. If you fail the test, they hit you with the banhammer. Psychiatrists would be cheaper and better, but democracy produces popular myths, not rational plans.