Hey!
Actually, this is being optimized right now! I'm taking a 2 week break from STM to work on a personal project, and handed off STM to a pal to optimize and clean up some garbage. STM was originally made for smaller lines of text like dialogue boxes or UI, but for larger fields or several instances of STM I can imagine this starting to cause problems on some devices.
Either way, Rebuild() being called under OnEnable() is kind of just a failsafe, I suppose it doesn't actually have to be there? I can just imagine it causing unintended behaviour if it wasn't there, so... that's why it's there. I'll forward this to my pal who's working on the GC optimization right now!