Even if you "cheat" by hitching the weight, you've still lifted it.
You can't argue that in the deadlift.
In benchpress it's whether the ass is lifted, if there's bounching on the chest(belly), et al. Some guys who are real fat, can arch and lift their asses so much that they barely won't have to lift if they want.
In the squat you can do a half squat, or a quarter squat, or even 1/8th of a squat.
And still claim you've "squatted" X amount of weight.
But in the deadlift - you got to lift the weight.
It's that simple.
So it's not a truer test of strength than the other two.
But it is a better one, because it is so much easier to avoid keep simple.
It's just grip and lift.