They don't as long as the start-up isn't tech driven, if you're building something that's fairly standard and doesn't require any complex tech behind it then yo

They don't as long as the start-up isn't tech driven, if you're building something that's fairly standard and doesn't require any complex tech behind it then you can go to a webdev company and they'll build it for you. However if tech is a core component of your offering (which it is for most YC companies) then product development needs to be guided by technological complexity as well as user demand. Which means you need someone who grasps both the tech and business side of things. And if someone fits into that category why would they work for someone-else's startup for a relatively small wage rather than working for a more profitable company (higher pay; more security) or starting their own company ? - essentially offering significant equity is the only way to hire these people. Hence they need to be co-founders.