I always thought that it was because of the average femur lengths for various height persons. I don't think that as we move from a short person to a tall person that femur lengths and other body parts proportionally get bigger in a linear way.

Also, if you kept the steeper angles, the CG with rider will be shifting further back on the rear wheel which might produce some stability and steering issues. Some bikes did change wheel base as they grew but other didn't.

At any rate, that's what I've imagined and gleaned from sources I've read over the years but don't even know if they were credible. So TIFWIW. Probably less than 2¢