I understand how the fact that a AVTuber is single adds a lot to the fantasy for their viewers, whether it's because of an illusion, fantasy, Yuri illusion, jealousy control, "unicorn" factor, and more. It's safer to have that fanbase if they don't have a (public) partner, which is why most go with that.
Personally, I prefer that, not because I don't want them to have a partner or believe they shouldn't, I just don't want to know the "lucky one" who has the real rights while the viewers are faping for their account. I think that breaks the illusion and a bit "cuck" fetish on it. (also if the dude moans or talks a lot/cringe porn talk, turn off for me)
BUT sometimes I think it's more interesting when it's "the real thing" when they're with just toys and vibrators become a bit stall/boring, "it's not the same" and I would like to hear them (maybe not see them) with the real thing having real sex, depend a lot of each AVtuber, some are nice just for "fap factor" and don't mind B/G, others are more "personal" that you don't want that.
Again I think CottonTail still my best example of a good format (the dude talking is my only problem) I wish others follow. personal opinion.