It's something I've genuinely come to dislike about vtubing, it's so overdone that it just feels cynical. I get that hyping up actual events like birthdays etc. makes them more profitable (as opposed to when a lot of fleshtubers stream on their birthday, where a handful of people say "happy birthday" at the start then they'll just play a game or whatever like normal), but the problem with audience hype is it has to be carefully managed. If there's a new announcement every week, then an announcement is no longer exciting, you're just turning your audience into stoners who've got comfortable with the high.
It's the same reason why new outfits should be spaced out properly, even for vtubers who can afford regular model updates.