Some more interesting topics around the subject that I haven't seen covered in the comments: - Hololive is huge and they definitely take a decent cut for their efforts. But why do people sign with them? The short answer is that they're a well oiled machine where people go through a sign up process where they create a test video etc. If accepted they're placed into groups (such as Hololive English) etc., but at the sa…
I think it's more about market dynamics than strict rules. There are clear benefits to collaborating within the same agency because it helps introduce audiences to other talents under the same umbrella. Everyone involved has a chance to grow together.
Once you start collaborating outside the agency, you're inevitably splitting viewers' time, attention, and even spending across different groups. Since every fan only has so much time and money to go around, those collaborations don't always make business sense.
That's not to say they never happen. If a collaboration is expected to bring in new audiences, significantly increase exposure, or otherwise benefit both sides, then it's much easier to justify. But without that kind of upside, it can end up helping another agency just as much, or even more, than your own.