I find it plausible in a narrow sense of “write” that doesn't mean performing the substantive work, but merely assembling the formal written presentation of that work.

It depends but I don't really view that as full on cheating then. It'd be nice for them to actually write a bit of it (otherwise, they're going to have a tough time writing papers once they're out of grad school) but the actually bulk of work is the real thing.

Also, you can't cheat on a defense. If you can, then your Ph.D is worthless and the institution you got it from is near worthless.

Reread learc83's comment and it's really true. You can't just turn in your thesis. Moreover, you have to defend it, and your committee will tear into it even on the minutiae (happened during my defense) so it doesn't matter how well you know it if it isn't your work because if it isn't your work, you won't be able to defend it. The only conclusion I can see is they are crap profs/schools but that doesn't square with the "famous people on committee" bit.

Even in PhDs there are people who slip through the cracks.

I knew a PhD who in a postdoc would routinely plagerize work (lifted verbatim from other papers, passed off as own work). I would be shocked if they didn’t do this to some degree in their thesis too.

This was from a relatively good university in the UK.

Sure some people plagiarize, but plagiarizing an entire dissertation is a much different. What are you going to do when you have to defend?

Also it takes a good bit longer to get a PhD in the US compared to the UK. In the US most STEM PhDs take around 5 years to finish--that's a long time to fake it without anyone noticing.

Don’t know about the US system. But I can imagine it can happen, at least in the UK system which I’m farmiliar with.

External examiner who is in a related, but not same field for example. Or they’re just a buddies with the advisor and only give softball questions etc.

The defense in the UK is not seen as a big deal, I don’t know anyone who has failed.

I can't imagine someone who isn't very familiar with their dissertation successfully defending here.

Also to even get to that point were the advisor thinks you're ready to defend would require constant back and forth your advisor. Then committee is going to grill you. And finally the vast majority of dissertation defenses end with recommended changes. It really would require full time devotion on the part of the ghostwriter, and excellent acting on the part of the candidate. Not to mention familiarity with the thesis to the point that they could have just written it themselves.

I can see someone helping out a lot with writing it, but there is no way someone could just do your dissertation for you at a reputable US school in a STEM field outside of some very rare fluke. A person doing this 2 or more times with different SOs--someone is lying.

Many people in the US have a low opinion of UK and Continental PhD programs. Perhaps this is part of the reason?

I think you’re correct in your last point (and others, though I have no direct experience).

I guess I’m skeptical that any qualification is 100% effective at evaluating/training (but that possibly says more about my own biases).