It's because there's not enough work when the startup is first building their project to fill up a pure designer's time, and if one of your cofounders is sitting idle while you're trying to build a company, you have a serious problem. Same reason there are very few tech startups with legal cofounders, or accountant cofounders, or techwriter cofounders.<p>You do get a fair number of startups with designer/frontend-engineer cofounders (37signals?) or designer/business cofounders (Apple?). But a startup founder needs to wear many hats. If you define your specialty narrowly, you won't be able to do everything that needs to be done to get a business off the ground.