why don't games have a dynamic quality/constant fps setting?

[hahher]

currently games run at constant quality and fps changes according to scene. why not design game engine so that fps is set and games drops or raises quality to try and maintain the set fps value?

 

[BFG10K]

The Unreal engine supported that but most games simply don't bother these days.

 

[Mellman]

i think playing a game with that would be weird, its like an opposite kind of lag....i mean if your gpu cant handle something now, your used to it skipping, if all the sudden things got blurry, i think my eyes and brain would flip out from being confused!

 

[high]

unreal does it when you set it in your unreal2k4.ini