There is no universal LEGO Technic set that contain each and every gear LEGO has ever produced.
None1 of those elements are out of production. You may not acquire all of them in a single set, yes, but they are still pretty much available in sets from 2021-2022.
1 Except, maybe, Technic Driving Ring 2L. But it isn't something that cannot be fixed by buying ...
Because the builder had those parts, and knew how to use them.
Perhaps those parts weren't out of production when that was build. There are extremely few MOC's that are updated when the parts go out of prodution. Perhaps the builder doesn't even have anymore, and their interests may have shifted. It does limit how many others are able to build a copy, but ...
I don't know if this should be an answer or a comment. But:
Because it works? That's kind of what the pieces were intended for, and they are put to good use. I, for one, could not have designed it any better.