I wouldn't exactly say that. They don't push for high-end visuals, but it doesn't necessarily mean they don't push the hardware. The reason why most MMOs try for mid-end graphics is simply because having 100 players casting spells at the same time will push your hardware.It's not that you can't it's just that people into hardware that pushes the envelope are generally mainly interested in games that push the envelope, something which MMORPGs don't do so it's a bit of a miss-match.
(While most extra-good looking game will focus on your main character and some enemies in a somewhat enclosed area, the end-game of a MMO usually feature open areas with the number of players ranging from 10, 25 to 80+ players together.)
That's why Age of Conan (MMO), a game that actually focus on great graphics, will go from gorgeous when adventuring alone to kinda-ugly when taking part in large sieges, as you'll simply have to turn down the graphics. (Unless you have lots and lots of money to throw at your PC.)
(If by "pushing the envelope" you only meant graphics that entirely true, but it was just a little vague.)
You want my blood? Come then, dog.I thought I smelled the stench of alliance pig.