5/8" bars used to be available, back when EMPI made good parts. So, a very long time ago. The 5/8" bars were a good compromise between ride quality and handling. I made a front one for my Spyder. It is made from an old 911 through-body bar. It works well.
The world today is pretty much on the "bigger is better" and "extreme" track. Sometimes more is NOT more.