It depends on your major... it's really tough to make it as a Biology PhD. There are just so many of them.

It depends on what you mean by "fail". If it means "drop out before getting a PhD", then you are surely right. But that's not how most people who enter PhD programs in sciences define success. They want a tenure track position afterwards (possibly with a few years of posdoc positions in between), and eventually they want to become tenured professors. And I suspect--I can't back it with any numbers though--the failure rate for them is of the same order of magnitude as for startups.

Mine is that it's spectacularly easy to fail at grad school...tons of my classmates left college figuring they'd get a Ph.D and are now dropping out after (or sometimes just before) getting their masters.

You don't so much fail at grad school as fail at finding lucrative work afterwards.