There's probably a selection effect at work. Would a highly moral person with a capable and flexible mind become a full-time moral philosopher? Take their sustenance from society's philanthropy budget?

Or would they take the talmudists' advice and learn a trade so they can support themselves, and study moral philosophy in their free time? Or perhaps Givewell's advice and learn the most lucrative art they can and give most of it to charity? Or study whichever field allows them to make the biggest difference in peoples' lives? (Probably medicine, engineering or diplomacy.)

Granted, such a person might think they could make such a large contribution to the field of moral philosophy that it would be comparable in impact to other research fields. This seems unlikely.

The same reasoning would keep highly moral people out of other sorts of philosophy, but people who don't have an interest in moral philosophy per se might not notice the point. It's hard to avoid if you specifically study it.