If you're an indie chuuba, there's literally no point to doing a debut, since there'd be nobody to direct traffic to you. If you're part of an agency, the debut is necessary to establish your character as part of the fold. It'd be pretty fucking weird if somebody just started streaming under the hololive banner out of nowhere, and people would be less inclined to just randomly watch them without any context. People are more inclined to watch debuts of characters in agencies they follow, which makes it easier to establish them.
For example, Aqua is one of the most subscribed people in hololive, but I've never bothered to watch a second of her content, since I have absolutely no context as to what she's like, and my time is already being spent on other chuubas. Meanwhile, Reine is the second least subscribed, and I watch her content frequently, because her character stuck out to me in her debut. I had time to watch an hour long introduction for Reine, which established enough interest in her to make room in my vtuber watching habits for her, but I didn't have enough interest to dig through already pre-established content to find a good starting point for Aqua.
