Because the way you're recommending treating them means they come out with no job skills, atrophied social skills, and otherwise ill-adapted for society. Thus,

Because the way you're recommending treating them means they come out with no job skills, atrophied social skills, and otherwise ill-adapted for society. Thus, they'll turn to the only tool they have, and they end up right back in. Your method is far from a deterrent, it'll just encourage recidivism and lead to ever more populated jails. Treating people humanely ends up being a lot cheaper in the long run.