Not quite true.
First of all, your arms mobility is already quite restricted, even by a simple breastplate. Vertical ridges and bulges in the middle of your chest or belly do not additionally hinder your mobility all that much.
Second, all armor is a compromise between protection and mobility, so some loss of mobility is acceptable. Bulges in the right places can deflect blows away from vital areas. Ridges make the armor stronger, and better able to distribute impact. This is the theory, and practice seems to confirm it.
And third, try google. You will see tons of pictures of historical armor, and you will notice that vertical ridges from neck to groin are common in all periods, and that a bulge over the plackart is often seen in late period armor. And before you claim that it's for the wearer's beer gut, remember, the plackart goes over another armor layer, one that does not have the bulge. It is for deflection.