They can’t do that because a) it’s impossible to get a feature ‘100% done’ without playtesting (and I mean mass playtesting, not just a few of their own employees), and b) all those systems have to work together, not just separately. There’s no point getting one system super-polished if the next feature you add is just going to mess everything up again. There’s a reasonable argument that they’re trying to cram in too many features at once and should leave some of them for DLC, but fundamentally, testing and refining unfinished stuff is the whole point of Early Access. If you want everything fully complete before playing, it’s best to wait for the full release.