I'm not a Tor developer, but I follow the mailing lists somewhat.
IIUC, every unique exit policy must be propagated out in the consensus. Adding thousands of unique exit policies (one per "custom" exit node) would make the Tor consensus grow quite large, which would slow down the entire network. I *think* (not entirely certain) that end user nodes have to retrieve the consensus before establishing circuits, so it would slow down Tor startup for all users.
If I understand correctly, then it wouldn't scale for every exit node to pick its own set of allowed IPs.
Besides, how would you decide what networks to exit for? Just BBC? Do you want to allow CNN as well? How about Wikipedia?