1. You might not have a good GPU.
2. You might not WANT a good GPU, if you use the computer just for work.
3. You might have a good GPU, but really "don't give a fuck" if you can see a shadow, a transparency or FX. Maybe you want it to be as fast as it can be, and you don't want compositor to take resources.
Why should anybody want compositing?
Better, why you suppose everybody should want compositing?
As far as I know, free software is about freedom, and choice is part of the concept of freedom.