Focal plane vs. leaf shutters in MF SLRs

I was cleaning my RZ and began wondering why each lens must incorporate its
own leaf shutter, then began wondering why we find leaf shutters only in
lenses. I understand why the shutter has to be in the lens for rangefinder
systems (proxmity of wide-angle rear elements to focal plane = insufficient
space), but why couldn't they be incorporated into SLR bodies?

In current leaf-shutter systems like the Bronica ETR and Mamiya RB/RZ, doing
so would make the lenses smaller and lighter. In focal-plane systems like
the Pentax 645N, you'd get flash synch at any speed.

I'm sure there's a perfectly good explanation I'm overlooking.What is it?