There's a lot of rumours flying around that HL3 might be coming soonish, and when it arrives it'll almost certainly support RTX. It's also possible that it'll be built on an evolution of the HL2 engine. If so, it might have been seen as a useful development and marketing exercise to backport the RTX changes to the older HL2 engine version.

Also hobbies. Not every activity has to be for monetary gain. Or full time for that matter.

Maybe not in this case, which may be a paid for nvidia ad. But in the case of a lot of mod/remakes, yes, they're done for free.

I would assume that nVidia financed this as a tech demo for their stuff. They did a similar thing with Quake 2 RTX a few years back.