Because the open community is apparently incapable of making a system with good UX. Try it and you'll get a bunch of people complaining that inline images are bloat and who needed an auth system anyway and why can't I use my IRC client from 1996 and your client wasn't supposed to use my RAM, I'm saving that for guests.

If and when those open discussion platforms manage to match the UX of Discord, I'm sure people will be happy to switch. But I'm not sure it'll ever happen.