Where's the 3D On Slow PC's?

With the latest programming technology and libraries, how come is it that 3D now requires the latest top-of-the-range graphics cards? I'm not just talking about the latest commercial games, but open-source projects. I run a bog-standard 600Mhz PC with no special graphics card, and I can't run any modern 3D game, apart from the ones I write. Half-Life (1) and Quake run vert smoothly without a problem, but for some reason anything written in the last 5 years either runs like a dog or not at all. It's one reasons why I make sure all my 3D games can run on the lowest spec PC. Why should we lose out on all the fun?