Because it's much cheaper.

So now you don't have to pay people to do their actual work, you assign the work to ML ("AI") and then pay the people to check what it generated. That's a very different task, menial and boring, but if it produces more value for the same amount of input money, then it's economical to do so.

And since checking the output is often a lower skilled job, you can even pay the people less, pocketing more as an owner.