/vt/ - Virtual Youtubers

Sorry for this let's say stupid question, im pretty new in this Hololive/Vtubers thing.
But i want to understand what's the point of this "Graduation" thing? Is it because it makes lots of money when they do a final stream?

I mean can't they just for example give Sana or Coco to someone else?
Ok her voice will change and songs to, but why can't they do that?
Aren't the girls behind the Vtubers just acting or do they have any involvement into creating the Vtuber?

Sorry for my ignorance but im pretty new into this, so don't know well what goes behind doors.