A big part of it was that they were definitely further up a year or two ago and have been trying to fire sale out manuals as they go. The website has been updated infrequently so the number is not as large anymore.
My contributions are involuntary, Jason. They're in some of those ROM images being freely traded about these last couple of decades.
Unfortunately I never had any contracts offering me royalties on future republished or derivative works unlike some of my peers. Not that they could claim any these days, anyway. But I think it still should be put in the discourse for posterity.
Since there is no legal way to buy the ROMs, most arcade machines have been recycled and trashed by now, it isn't possible to buy the original arcade machine, and the only way to play these arcade games now is via MAME, it seems as if piracy is the only way to get ROMs until someone gets permission from ROM developers to sell their ROMs.
MAME has thousands of ROMs and games, it would be near impossible to get a license to sell each and every ROM.
StarROMs tried that with the Atari arcade ROMs but failed.
There's multiple bits of objectives here - education, research, practice, and awareness. As time goes on, it's obvious that distributed sharing of data is the only real vaccine against the inherent problems of the modern Internet, and learning more about different ways this might be done benefit all.
Plus it's nice to have some of this data scattered about the world.