There is nothing preventing apps from providing a registration flow. For example Voyager has it. I suppose the problem is again which instance to choose for signup. You can discuss this in lemmy.world/c/boostforlemmy or lemmy.world/c/lemmyapps.

As for multiple Lemmy apps being available: Most of the current Lemmy users came here in 2023 when Reddit locked down the API and killed third-party apps. Thats why a lot of apps are now available, and everyone can decide for himself which one he prefers.