It's probably even simpler - the conviction is the deliverable produced by a prosecutor, and the thing that moves their career forward. So they are incentivized

It's probably even simpler - the conviction is the deliverable produced by a prosecutor, and the thing that moves their career forward. So they are incentivized to always go for the harshest possible punishment, in the same fashion that a company chooses profits over everything else. The system isn't malicious, it's just not designed to counteract this sort of thing.