Because the impact in optimizing hardware like that can be not so important in many applications. Getting the absolute most out of your hardware is very clearly important in game programming, but web apps where scale being served is not huge (vast majority)? Not so much. And in this context developer time is more valuable when you can throw hardware at the problem for less.
Traditional game programming you had to run on the hardware people used to play, you are constrained by the client's abilities. Cloud gaming might(?) be changing some of that, but GPUs are super expensive too compared to the rest of the computing hardware. Even in that case the amounts of data you are pushing you need to be efficient within the context of the GPU, my feeling is it's not easily horizontally scaled.