I have a very good friend who began playing pro ball in 1939, and ended up as the pitching coach for the LA Dodgers from 1968 until he retired in 1980. The way he describes it is, a pitcher is storing energy in the windup that can’t be stored in the stretch.
I think that may be the issue.... Obviously there is no way to "store" the energy.
Today's technology allows us to see that there is no mechanical advantage to the windup. Some pitchers are more comfortable from that motion, or mechanically do things differently which impacts velocity.
One of my son's pitching coaches pitched in the majors for a few years. He said that one of the things the pitching coaches worked most with him on was trying to get his velocity from the stretch to be the same as his velocity from the windup. In his case, he took a shorter stride, didn't get his hips fully engaged, and changed his arm path in the stretch. Once those issues were corrected his stretch velocity was within 1 MPH of his windup velocity.
When he first started working with my son he clocked his velocity and didn't see a difference between the motions, so he had him just work from the stretch. When I asked him why he said, "Too many moving parts, not enough benefit".