Or maybe they do and I just don't know it, but I could've used one today.
We upgraded the service on a 1972 house from 150 amp to 200 amp with a larger panel to allow for a large addition the homeowner is planning. While labeling the new panel, we discovered that the entire upstairs - 4 bedrooms, 2 bathrooms, the attic, hall, and a whole house fan were on one 15 amp circuit. This circuit was on a GFCI breaker, I guess to protect the bathroom outlets, but we had to replace it with an AFCI to comply with 210.12(B). If AFCI's were also GFCI's, then that would be the end of it, but we now have to change the bathroom outlets to GFCI's. I'm actually going to try to sell the homeowner an upgrade of the bath outlets to a 20 amp circuit and splitting the rest of the circuit into 3 circuits, but that's beside the point.
If an AFCI can detect a ground fault, why don't they make them compliant with AFCI and GFCi rules?