Because of the security and risk implication of not preventing it. Goldman Sachs is both an investment bank and an asset management firm, literally the same company. However, they have clear protocols in place to separate the two sides because otherwise the opportunities for impropriety are too high. I could see a similar argument for separating Facebook and Oculus accounts.
The fact that the two components of an analogy are not the same thing is the point of the analogy. If they were the same thing, the analogy would be useless.
In this specific analogy, the point is, despite having different business models, society benefits from either conglomerate separating their respective two aforementioned business units.
No, their businesses are managing money and taking companies public. They're overall revenue would benefit from aligning those businesses, but they aren't allowed to. They are kept separate because of the risks of having them together. The Facebook scenario is a different (but comparable) situation.
