Also why are the nuts round rather than half or 2/3 round? Looking at them it seems to me that raising the nut higher and cutting off the top of it would improve the trigger pull. I'm picturing the bowstring catch say 1/2" above the nut axis while the sear is say 1" below it.
Nuts are round because they typically bear on their cylindrical surface, rather than on an axle. The socket in which the nut rotates is inlaid with a heavy piece of bone or antler, and it is this which bears the "weight" of the spanned bow.
Does that make sense, or should I try explaining it differently?
If you make it to bear upon its axle, you can make the nut be any shape you please, but the axle, and the places on the tiller that support that axle, must be strong enough to do that job.