That's true but on the other side, ideas are sold dime a dozen, only their successful implementation proves in hindsight that the idea was worth something. Stealing ideas doesn't mean much as they are available aplenty but a startup which successfully implemented that idea is worth something.
Ideas can be promising but implemented sub-optimally. Using this method, they get inside information on where things can be improved. Then they have the financial clout, the data analytics and the resources in place to create a successful final product.