It’s really simple. They realized they could make more money by never releasing it. That realization has made them more money than the vast majority of game studios. It’s a glorified ship showcase simulator, that’s about it, not really a game. If they set a release date they’d reduce the amount of money they make, why would they do that?
I really don’t think it’s a conscious choice. Just Chris Roberts’ typical perfectionism and nobody there to say “Ok, this is enough, we must ship something”.
Having said that, I used to think Elite Dangerous had the superior model, but they’ve proven me wrong by going to absolute shit with Odyssey (no VR support, no walkable ships) and pretty much dropped all development thereafter.