Why can't we just open one subscription to two or three relays of each followed person in order to build the default feed?

If that was possible that would make implementing "outbox" even easier than implementing a client that just uses a static set of relays. Caching notes from is easy, it's easy to continue fetching from where your cache ends. Relay selection is easy, no need to build complex filters aggregating multiple requests and do fancy relay selection based on whom you're following matches whom. Everything is just one loop.

Nostr is easy again, developers are happy. Even the dumbest web developer can do censorship-resistant Nostr clients for everything.