Why dont we mount mobo’s sidesways in cases?

That way: 1) the gpu doesnt sag. 2) the airflow of the gpu fans are in the same direction as the case fans. Seems kind of weird to me that no case manufacturer(i know) does this. Am i missing something obvious?

Edit :Way too late with this, but i didnt mean horizontal cases, i meant turning it 90 degrees in your vertical cases.