Same reason why there are so many text editors that all do basically the same thing, I suppose. More options are not a bad idea, IMO. Maybe they'll do something different and interesting.
A text editor is a whole different beast than cloning Windows 95/98/2000. I will note that ReactOS isn't simply building a compatibility layer, their end goal is to allow users to actually run unmodified driver binaries from that era to get hardware working.
Some win95 / win98 software doesn't work all that well on Windows 10/11. It's mostly stuff that was targeted for the early releases of Windows and cannot be run in DOS (or the DOS version is more primitive than the Window aversion).