It's funny, because if this wasn't the east...The manager would become a meme itself.
Seen it happen a few times. (Heck, one could even argue that even if Yagoo isn't the manager the comparison it's appropiate)
Fans even end up asking for the manager to appear on stream from time to time. But because eastern fans consider their streamers to be well... idols, the presence of a man or bf ends up tarnishing their image of their... imaginary gf?
Now, as some of the comments have established, Yuu does have quite the experience in live-streaming so it makes sense for him to be a manager. That would answer the question of "Why does he sound so young?" for part of the fans.