1. There's nothing stopping them. That they don't isn't part of the protocol.
  2. There are relay directories, note the plural, and there are relays that aren't part of any of them, note the plural.
Nostr isn't a centralised, solve your problem, profit making enterprise. There are rules, but frankly after NIP-04 it's all "make it like Twitter" bullshit.
It isn't Twitter, it isn't federated, it's going to work out its own way in the world and the apocalyptic predictions of silos, hoarding, may be right and Nostr isn't the solution. Making it like Twitter though is definitely not the solution.
Jameson Lopp doesn't know how many followers he has, he doesn't know now how many people read his profile and don't follow.
The easiest solution might be to make a relay that follows the relays you've added to your client, then others could follow that one.
PS. Note to self, read then respond. That way I'd find the idea has been implemented; #134918