So did most people just stop PC Gaming around WinXP or something? Online most of the games I hear about are either A) Win98 games that just happen to still work in XP or else B) Windows 7 or later games that just happen to still work in XP. It's like the XP era itself is a black hole.

From my own collection I only have found the following:

Doom 3
Medal of Honor: Allied Assault
Echo (an adventure game that I think takes place in prehistoric times)
the Myst series--I think Myst IV and V require XP
Call of Cthulhu: Dark Corners of the Earth
Warcraft III

I'm not sure how many of those are "actually made for XP" vs "Just happen to work in XP."