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?
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.