I was thinking they must do this due to the way the anim blueprints are compiled. They do some kind of validation with bone names and bone indicies, and things like that.
If I was them I’d change this underlying behavior though. It would be great to have a common anim blueprint and just specify skeleton and anim assets to use. I guess there’s an assumption that your characters will always be humanoid. Imagine making a game like DooM though with all sorts of types of monsters that are going to most likely have very incompatible skeletons.
The funny thing is, you can write C++ AnimInstance subclasses that do this same thing. Only Anim Blueprints are tied to a skeleton as far as I know. I wonder if you can build a native animInstance by manually linking anim nodes together. The examples I’ve seen of native anim instances do very basic things like control bone locations. Maybe you can prototype an animation blueprint and once you know it works, manually convert it to a native blueprint using the same node layout. Not ideal at all of course, but it’d be interesting to try…