Because they are so fcking hard. As well, as you get tired in any exercise form gets rough, and deadlifts are an exercise where good form is particularly important to prevent injury; other lifts have a bit more tolerance. If every rep were watched by a competent coach, trainer or training partner, this wouldn't matter, but most lifters lift alone.
That said, when I write routines for beginners, if I give them deadlifts, then I give them more than one set. I mean, they're starting on 60kg, they'll get to 80-90kg before they come to me for a review of things, and I'll see them a couple of times in the meantime. 60kg for a couple of sets is not going to kill them.
And then with PT clients, obviously I judge it on an individual basis. I have a woman who's over 35 and around 50kg, she's not going to deadlift 60kg even for 1x1, we need to do other stuff first.