What evidence to the contrary? 1% of the US population does commit over 60% of violent crime: https://pmc.ncbi.nlm.nih.gov/articles/PMC3969807/
I mean, trivially, using our brains for a nanosecond, what if that 1% of the population is almost always 16-18 year olds when they commit those violent crimes. The 16-18 demographic is roughly 4% of the US population (Google). That would mean locking up 1 in 4 high school students for 6-20 of their most formative years, and thrusting them back into society with a “Mission Accomplished” banner hanging behind you.
Play with the numbers a bit (maybe it’s 1 in 20), but the point stands. Using imprisonment to try to quarantine a demographic that is perceived as irreparably violent is a barbaric, sophomoric idea that has very little evidence of success in the modern era.
The reason I brought up youth is because, unsurprisingly, most violent crime is performed by people who don’t have a fully-formed prefrontal cortex. Feelings of invincibility and a sense of not having much to lose.