Yes, anybody who is good at their job is likely to be able to get a better (paying) one. But that itself doesn't incentivize DAs to aggressively pursue cases.

The incentives they actually follow (and there are plenty of them!) are much simpler: municipal politics (not wanting to be seen as "weak on crime") and power dynamics (wanting to come down with the "full force of the law" at every opportunity, as a disincentive to future criminals.) Plus, plenty are just plain cruel.