Because they are betting on founders not the product, product will change based on market response to it remember segment?what it was when it was initially funded and what it became after pivot. In a way it is similar to asking why do you want to hire developer who does not have experience with xyz tech? this doesn't looks outrageous as we know experienced developer will eventually be able to learn and that is why we are hiring him/her. This is similar essentially we are betting on person not product/technology.