Doing a good job as a manager is hard. You need to triangulate between a lot of disparate opinions and viewpoints, negotiate compromises, and help your team develop and advance without hurting the larger group's goals. It takes empathy, good communication, and the ability to squelch your own opinion when circumstances call for it. None of those attributes are universal in engineers.
In fact, just about everything in a startup is hard: sales, recruiting, design, marketing, etc. Pretending otherwise or asserting that only engineers have something special to contribute is IMHO toxic and short-sighted.