Ah, but there are forces that keep people from contributing, or at least increase the amount of effort required considerably. These forces are not evenly distributed, and there are several dynamics operating that actually increase the effect many of these forces have on members of underrepresented groups.

So, given that this is to some extent a self-reinforcing phenomenon, if we want the community to look like it welcomes all comers, we not only have to be welcoming (which we aren't doing that great on in the first place), but also do everything we can to counteract the force of those feedback loops that keep groups underrepresented.