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.
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.