>>> If you do the time and work for your professors you will get the reward. There is no risk.

1. Your experiment fails to produce a result after a few years of effort (my project, we don't know to this day what went wrong, and I was lucky to find a new project).

2. Loss of funding or institutional support. (A large program at my state's university pulled its support for a process that required regulatory approval, and an entire group of faculty and students all had to leave.)

3. Your advisor quits, changes jobs, gets fired, goes to prison, dies. (Many cases).

4. Your advisor holds your thesis hostage until you publish a certain number of articles (a friend of mine, she sued and won).

5. Mental health issues (high incidence of clinical depression).

6. Personal animosity between members of your committee (another friend).

How these risks instantiate themselves is that you have to start from scratch, often with a completely new research project, and finding one isn't guaranteed by your department. You are almost completely at the mercy of one person -- your advisor. There is virtually no oversight.

I agree, those are all real, especially the advisor and committee.

Most of these are factors in any employment, and I would argue things like chance of losing funding at your job is worse than academic funding threats.

Perhaps a higher likelihood but also a lower cost to the individual. You presumably work for a competitive salary. When you lose funding you presumably jump employers for a comparable position.

In comparison, most PhD students work for a very low salary on the expectation of a payoff after something like 3 to 6 years. Framed that way, being forced to either start over or depart is incredibly costly.

I wouldn't say it's about "failing to meet the standard". Sure, there is no exit exam you can fail, but there are still people dropping out of a PhD.

It could be because you realize you don't really like research - that involves reading and writing a lot of papers, going to conferences not just tinkering. It could be because you had the wrong professor who failed to lead you and left you by yourself. It could be because you gave up at a low point, where most PhD student go through. It could be because after 4 or 5 years your professor keep saying "you're not ready yet" (I've seen that in humanities).

So it's not really a problem of "not being good enough", but it definitely happens.

Definitely true. Attrition is real, but "risk" is probably the wrong term. It's one of the lowest risk options available for young people.