Oddly enough, the collabs for OTHER regions were their own characters and not skins.
Part of the reason why is, to my knowledge, is them fearing of fucking up with the meta. Back on the Korean server with their collab (with Guilty Gear), they made one of them, Millia Rage so OP that everyone and their mother that got her dominating everything and might've changed that server's meta. Japanese server collab (with Code Geass) has a more lukewarm/average characters that doesn't break the game
and they get some skins too (if i was right)
The other one, which I'm guessing, is that they just didn't really want to try developing completely separate characters during the server (Global region) infancy. Considering that they have:
- a eldritch being disguising as a human that can sent out swarm of tiny monsters at her call
- a magical woman with her own phoenix and fire related magic
- a loli with huge melee weapon
- bubbly, happy go lucky character with summoned cuties that has guns/fire projectiles
- and a floating horny bait woman with power of influencing death (in this case, immortality buff)
They probably figured out, like, "hue, let just make the vtubers as skins for our existing characters, ez peasy". No need to care about fucking up the PVP and PVE meta being ruined by new characters and at the same time doesn't need to brainstorm for character skills and stats anyway.