This is the only answer to the OP's question. It's not about company rules, idol rules, or whatever people want to think it is.
They will do it if they want to, and have the time to. You can't just put two people (or more) together and have it work. It's not just in this industry, but any professional industry. All the talents want their streams and content to be top quality, worthy of the companies name.
Mori mentioned very early in her streaming career that she would love to collab with more people, in Hololive, outside of Hololive, in different industries, but it can't "just happen". She wants to know the other person, know what topics to chat about, games, life, etc. so that the stream isn't an awkward situation. This takes time.
And we all know what kind of 'free time' the talents have outside of streaming, with music production, training, marketing planning, and all the un-announced projects, so this is pretty much why collabs aren't as common and rare. And when they do happen, its the same people. It takes time to plan and know the other talents. It's simply not that easy.