The reason for that, is to avoid having influence from outside. Even "angel" investment can be problematic, as the "angel" has the ear of the leadership.
I have found that even well-meaning outsiders can have highly destructive influence, because they don't understand the culture and they aren't the ones on the hook, if things go pear-shaped, as opposed to the ones that have a real, personal, stake (like all those Ello users, who lost so much).