I'm not sure whether they can or cannot keep up with spam attacks. You'd have to ask them and see if they even care. I'm sure some of them just setup a relay and don't watch it while others are actively developing their relay software.

When you click on a relay you can see who runs it. Some of the relays don't even have a contact information.

Yes it seems to be common. I see spam all the time on the global feed. I'm sure it will be fixed in the future. Nostr is fairly new.