They do, but a lot of the social good companies are not actually non-profits but more like low-profit/high-impact, or B-corp style. In fact a few states have Benefit Corp/Social Purpose Company LLC-like structures.

So it puts you in a somewhat weird spot since you will never be potentially worth a billion dollars (which is the guideline for the for-profit track of YC) nor are you a pure non-profit.

It'd be cool if they added such a track or loosened the non-profit restrictions so it's not necessarily a 501c3 structure.