For the same reason it doesn't happen with humans.

Even for AI, it's probably better off paying another AI that is specializing in something and done all the work rather then reimplementing everything from the ground up.

Is it specializing though? If both you and them have built your business on the latest Opus.. where is the specialization?

"From the ground up" used to be a moat, but if the LLM marketing materials are to be believed, Joe Lunchbox can slop-code a 95%-equivalent of any SaaS over a weekend with a $100 subscription.. so why would it ever make sense for a business to pay a non-trivial recurring expense for something they can do themselves?