There are lots of interventions that show massive returns on investment in social welfare: a recent one has been extended availability of support for teenagers aging out of foster care, that takes their outcomes from something like "percentage who have become homeless within one year of their 18th birthday" from 70% down to 30%, and similar for arrest records and pregnancy among girls.

But, sadly, many people feel morally injured by spending money to proactively help adults who should be eating their own boots or whatever, and so it is less of a sacrifice to spend 5 times the money on jailing them instead.