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Because streamers fucking suck at commitment.
All an anime avatar needs is to be a little less human and play into some fictional aspect, particularly one inherent in anime. 99% of vtubers already fucking stink at this.
A non-human character will require full fucking commitment to the role WHILE being entertaining. And I can name 0 in the current roster of all corpo and non-corpo EN vtubers who can pull it off. This type of thing is more suited for a JP, who have thousands of episodes of kids anime to pull reference from. And even then it's an uphill battle.