Because startups always have extremely limited time and resources, prioritizing the UI comes at an enormous cost to the company. Sometimes it’s the right thing to do, but when it’s not, when it gets top priority because it’s the challenge that the designer founder might be most comfortable with, or simply the one that he or she prefers the most, that can be disastrous. Because when you’re designing, you’re not necessarily acquiring customers, or marketing your product, or forging partnerships — or any of the many other complex, taxing and ongoing efforts that startups require, but which are only tangentially related to design.