Not only that, but getting a PhD is a whole process. You often still take some classes, and do a lot of research. All that time you’re interacting with other people in the same program, with a thesis advisor, with various profs, grad students and TAs.
Only at the end do you go off to write up your thesis, and most students at that point have a thesis advisor who they consult regularly. (Many of them wish they could spend much more time with their thesis advisor.)
If someone just magically shows up with a thesis at the end of that process, and they haven’t been consulting their thesis advisor (or what they’ve been talking about doesn’t match what’s written), or the thesis doesn’t match all the research and discussions they’ve had with people over the previous many years, it’s going to draw a lot of attention.
And then, after the whole thing is written up, there’s typically a thesis defense, where people who have expertise in very similar matters grill the student on the contents of the thesis. If you don’t know everything in the thesis (and more), you’re going to be in trouble here.
I’m sure there are various degrees of cheating. At the low end, you’ll just have someone who has writer’s block, and needs help trying to get the ideas from their brain into paragraphs. At the other end, you’ll have people who legitimately don’t understand everything in their thesis, and truly had someone else doing the work for them. But, just based on how the process works, it’s going to be very hard to get away with true all-out cheating.