I know; but I just think of *users*
I don't think the directory service can currently scale to match the same # of exit nodes as you have active users of tor.
I think arma was positing maybe it could scale to 5-10 thousand exit nodes listed in a directory service.
maybe that's been improved, or should be.
but I would hazard to guess many more people use tor than that.
in the same way routers already are challenged to keep route tables for all the other routers they talk to.
if instead you expected every router to have a direct route for everyone who used the internet?
it doesn't scale... ;-/
or well, it does but everyone using isn't everyone routing.