Well, the truth is that not all deadlift schemes are sets of 5 reps. If your question is “why do they stay in that range?” then the answer is likely because the deadlift is a strength movement primarily. 5 reps is the height of the strength rep range. You will, however, find people performing deadlifts in the 2, 3, 4 etc. rep ranges. You’ll even find people going out of the 5 rep range to work in the 8-10 rep range. It really depends on what you’re going for. I will say though, that the deadlift is seen as more of a strength movement and that I don’t really see people going very high in reps with this movement (unless they’re trying to hit a pr.)