The best designers are often frustrated artists that found comfort in design, where (perhaps subconsciously) they could use their talent without having the success or failure of their work fall 100% on them.
Being a collaborative process, design is kind of a shield from artistic criticism to some degree. If it fails, well it was the client's fault. But when you are known as a designer, and known as the founder, all the creative control comes down on you, which is akin to creating a work of art - with product primarily, but also the entire business as well. Everything you do subconsciously reflects on you as an artist instead of as a designer. Its scary. It can drive a creative person to madness, all of the things that go into a company, all the details that are completely fucking wrong all the time and you can't get control of any of them. Its like a painting with paint that never dries, and keeps dripping down the canvas. You constantly need to be painting or it looks like total crap, and it is hanging in the gallery, right now and everybody can see it. It makes me freaked out just thinking about it.
So while there are few designers who have made the leap to full-time founding entrepreneur I'm fairly certain that every designer has attempted to dip their toes into becoming the founding entrepreneur at least once, hit on this nightmarish reality, and then stepped back into the designer comfort zone with a sigh of relief...