Of course, you have to be insane. Many people are insane however without feeling compelled to write phd theses. What extra thing is it which is responsible for this particular form of madness? Maybe it is a kind of absurdity; but maybe it is a kind of moral degeneracy, of abandoning oneself to the seductions of Beelzebub.
I am not sure if there is a circle in Dante's hell reserved for people who write phd theses, but IIRC there is one which is modulo poetic licence the same as floundering till eternity in a lake of your own excrement -- or is it burning tar? I suspect that the sin which gets you into these conditions is the one which consumes the phd candidate.
Round this lake, your supervisor patrols with a bayonette. There may also be half-dead foetuses floating in it, and hideous ravening monsters that detect their food by its whimpering.
There is also the possibility that the thing will be dead on arrival, or too miserable and monstrous to let live. The examiners don't want to do much more than cut the cord, hold it up by its feet and slap its arse. Or give it a couple of kicks of a shit-stained abbatoire boot. Else it gets slung out for the crows.