Why no cases with 2 rear exhausts?

Cases always have a rear exhaust located near top, but why don't cases come with the option to install a second rear fan? Wouldn't it make sense to have a rear fan placed below the GPU and above the PSU? I feel like that is dead space. After all the GPU just blows air back into the case. I'd think it'd be great to have airflow pulling that out too.

Any thoughts?
 
Case manufacturers could at least leave the option to remove expansion slots in back to add a small case fan. The space under GPU seems like it just blows around and isn't directed. In my case I have 2 front and 2 side fans as intakes. The front would be enough but I need to keep the GPU cool and supply it with cooler air, hence the 2 side fans. So, there is some directional flow pushing towards rear of case from front....Just never sure why a small fan above PSU wouldn't make sense.