Because MAME is, on paper, primarily a documentation project. This is not always "convenient" for people who view it merely as a source of entertainment.
Not Working status means an incomplete ROM dump or incomplete emulation.
You can see why the various maygay systems are listed as not working right here in the MAME source code (relevant text in red):
"ROM dumping" is the act of reading out the game code from ROM chips on the game circuit board.
For various reasons it is not always possible to dump everything using conventional tools, in some cases highly specialized equipment is needed, and/or a special set of skills. It's better to have an incomplete ROM dump available than nothing at all.
Some games can still work when the dump is known to be incomplete. In many cases, as a temporary fix, the missing data is borrowed from the dump of another version of the same game.
Once there is a complete or better dump available the missing data is added. This can be confusing to newcomers because it usually means that a game that used to work no longer works because of missing files or renaming, as the ROM sets are rearranged.
Index 
Flat 
Re: What is "Maygay" and why is there so many "NOT WORKING" ROMs?
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