It's been that way for thousands, millions of years, and I think for many of us our brains may still be hardwired to that norm....maybe there is a good reason birth has historically been a women-only event?
I used to read about women having babies when I was a kid - which was before partners attending births became widespread, I think - and I dreaded the idea of having a baby, because of the missing partner aspect of it. Having the person I loved and trusted most outside pacing the floor while I was having our baby seemed...freakish. It still does. One of the many, many, many things I hate about c-sections is the enforced separation from dh (only while they place the spinal). It sucks.