A: Yes, because they are niche. Many are included in "non-merged" MAME sets under the hbmame folder if you obtain a full split set.
Notably, because some homebrew games were developed on inaccurate or outdated emulators, they might not run correctly on real hardware or even on MAME's highly accurate core. In these instances, HBMAME developers sometimes implement intentionally "inaccurate" or hacked drivers to ensure these niche games can still be played. hbmame roms
Brand new games coded by modern developers for vintage arcade hardware (like the Neo Geo or Sega Genesis-based arcade boards). A: Yes, because they are niche