Why?
I like a good extension ecosystem. For the mothership, GNOME, you can only implement one idea, maybe include a couple ideas but the boss or the group has to decide upon one idea. With extensions, everyone, even a maintainer herself, can write one. You do not have to talk to someone else. You can just do it.
As long as the api is well written, extensions are better than having one big mothership trying to accomplish everything and pleasing everyone. Imagine having an IDE without extensions. You have only the opinionated version of the main dev. With extensions, everyone can put his flavor on top of it without asking.
Edit: don’t ask me why extensions and especially extension manager isn’t included in GNOME itself.
And apple users had money.
Linux users still have money