It seems like every Lucas Arts game is for sale here on GOG except the one that I most want. Why isn't Indiana Jones and the Infernal Machine available?

I don't know the answer, but your assumption about every LucasArts game being on sale is false.

Most notably there are two Monkey Island games missing, as well as some other well-known games, not to mention obscure stuff. Even some of the classic games aren't here, like Full Throttle and Grim Fandango, although some might argue that they are not needed as the remakes by another company are here.

There are even Indiana Jones games besides the one you mentioned still missing from the catalogue.

Now you can start guessing whether it's compatibility issues, franchising issues, or some other boring legal stuff that is keeping those games in the vaults. LucasArts (=Disney) is the only one who actually know that for sure.

I am sorry to say so, but it is just too niche and it looks too much like a mobile game.

Your joking, right?
(I know that people do like to omit the emoticons nowadays and I misinterpreted posts twice lately so I am asking)

He's referencing Opus Magnum-gate.

Thank you. Saved me at least a bit one more time looking like a very dumb person :)

I took a second and used google for you:

https://www.google.com/search?q=site%3Agog.com%2Fforum%2F+Infernal+Machine

There's a wishlistthat you can add your vote to for this game:

https://www.gog.com/wishlist/games/indiana_jones_and_the_infernal_machine_1

This thread says getting it to run on the newer OSes is an issue:

https://www.gog.com/forum/general/wonder_if_gog_will_get_indiana_jones_and_the_infernal_machine

although another thread pointed to this fix:

http://www.markusegger.at/software/games/jones3d/instructions.html

Pod Racer isn't here either. LA isn't really big on maintaining their older games. They either work as they were originally released or you don't play them.

Whatever the reason for it not being currently available, the launch of the new Indiana Jones movie would be a wonderful opportunity to make this game available for digital download.