[throwaway5752]

Let's just say - there are some critical things you have to be good enough at and those correlate with commonly observed corporate structures. Some degree of Sales, Marketing, Legal, Security, R & D, and Finance competencies need to exist across the founders. Sometimes one person can do it, but they also need to know how and when to bring in experts if they are lucky enough to need to scale up.

On a very basic level, not having a cofounder in any capacity can create a structural bottleneck when two important things have to happen at the same time. A trusted second party provides resiliency.