same reason you can't play old DOS games without an emu like DOSBox, the old games were made in a time were like 512MB of RAM was EPIC or MYTHICAL, so basically your PC winds up being so fast (if they dont have vsynch which dos games dont afaik) the game can't run or runs SUPER FAST 900000000000000 FPS. dosbox, stuff like nes emus etc force these to run at 60fps so they are playable, afaik!
I read an article somewhere that all of the "unique effects" that old graphics cards used (like transparent pixels and really old shaders) is a short line of code that could easily be incorporated into newer GFX cards but the manufacturers of these things are too concerned with speeding them up than typing in a few short lines that slow it down. I can't wrap my mind around the idea that a 256mb graphics card in a system with 2gb of ram runs Vampire Bloodlines at max quality but the same setup with 4gb of ram forces the game to permanently switch to low. Why is it that all the magic effects in Planescape Torment, which happen to be sprites with a bunch of alpha transparencies, can't be "emulated" or whatever using that same graphics card?
UGH sometimes I wish I were these PC manufacturing bigwigs because I'd take the time to ensure that support for all systems is maintained *flowery ideology for another 5 paragraphs*