Cost is the big reason to use a dropout with an adjustable seat stay angle. One SKU for all frame sized. Minimal joining prep. And it works very well.

This type of stay attachment has been done for many decades, Raleigh's DL-1 (their rod brake 3 speed bike) might be the most common example of this (and this model was called by a Raleigh sales rep as being the most popular bike they make, if you include all the examples made in the 3rd world under license).

Cast Dropouts This is a link to Ceeway's site and about 2/3s of the way down the page are a number of "Articulated Angle" dropouts. Andy