The reason is that a startup is basically an extended learning process: its goal is to learn enough about the market and technology to bridge them, and then capture that knowledge in an organization. If you're employing a freelancer to build the project, then that learning is captured in their head, not in your organization. If it looks like you actually have a good idea (most people don't), then there is nothing stopping them from building your MVP and then choosing not to renew the contract, and instead going into business with a competing product that captures all the knowledge & experience they gained building yours.
This is exactly what Mark Zuckerburg did to the Winklevoss twins. While the latter did manage to sue for some small piece of the company - whose position would you rather be in?