And unsuccessful companies usually ignore their mentors' advice and fail (all the while scratching their heads as to why the mentors quit).

I can't tell you how many startups I've mentored where the CEO/CTO/founders would not listen to me or the other mentors/advisors on the team, and eventually it gets to the point where there is no reason to continue, the company's doomed to fail or stay stagnant in terms of growth/additional funding.