During my research internship, I met many PhD students whose labs simply ran out of funding. In some cases, they'd ended up having to scrap their PhD as the topic was too specialist to do anywhere else (e.g. requires a high level of biocontainment) and nobody wanted to fund it.

I think if you got three years into a PhD and were faced with the prospect of starting it all over again in another lab, it wouldn't take much to convince you to throw in the towel and do something else instead.