Of all things, nepotism seems like an odd thing to single out. Do you have any numbers for this claim? It would be useful to hear e.g. what percentage of people secure positions through referrals/family members.
But when you weigh by total wealth the Walton and Mars families skew the data in favor of nepotism. It also depends on your opinion of source of personal wealth of Ms Bezos or Ms Jobs (and anyone in similar position) - nepotism or self-made?
Evidence would be nice to have though.
As for me, I've had enough anecdotes to think it's plausible. Just recently we hired a new CTO at my company, and he got the job simply because he was friends with our CEO and he needed a job. Turns out he's completely incompetent...