Because giving up 1/3 of a company that will make you rich is better than your company failing.
You couldn't always get a $5 million valuation for your pre-revenue, pre-traction product. When it comes down to it, you have to take what you can get.
Because you don't have much capital or revenue when you're just starting out, you take what you can get. If you demand a minimum of, say, $5 million for such a stake and nobody is willing to give it to you, what then? Your unshakeable self-belief isn't going to magically make payroll or pay for your overhead or purchase That Thing You Need.