Why are startups suddenly branding themselves the "OS" for [thing]?

Because in reality, they don't have a no clue on what they are doing and they frequently compare themselves to Apple.

Cuz Investors love hearing how you can milk both sides - producers and consumers - https://en.wikipedia.org/wiki/Platform_economy

So startups tell them what they want to hear. When one gets funding everyone else copies the pitch.

Cloud AI OS...Oh my!