Because running a business is stressful and requires a certain amount of mental synchronicity/complementarity on mercilessly hard choices.

Someone you discover you can do this with, you can of course handle easy low stress times with.

A friend-first person on the other hand is usually someone you make non-economical choices with, for the sake of pure enjoyment. It doesn't translate well to the rigor of making money, and they may very well have different, incompatible ideas about how to do that.