It kind of sucks to properly expose AttributeSet creation to blueprints, because you need to set up lifetime replication in unique ways, require a repnotify function with macro inside for each attribute to guarantee that replication works properly, and the Attribute change functions pretty much require you to identify the attribute that’s currently being changed by checking up the UProperty belonging to them.
I am not convinced that it is impossible or even all that difficult to properly expose the creation of new attribute sets with new attributes within blueprints period, but I would think that it’s barely worth to do it considering it seems like such an annoying extra mile to take when the cpp code is so incredibly trivial(if obnoxiously wordy per attribute) to write, once you are aware of which macros to use where. It’s not like you will keep adding new attributes to your project all the time either, it’s the kind of thing you’re usually done planning and implementing relatively early into the project, with some minor tweaks and additions happening afterwards.