Possibly because there's a form to follow for the deadlift which has been developed to minimise the risk of injury? We get drilled in the efficient moving and handling stuff for work, lead with the head, try to keep your weight centered etc. (I'm not very good at it, have already got some lower back issues from years of performing restraints, so really should pay more attention to the whole thing). But yeah, it can be the little things done badly instead of the big things done well that knackers your back.