I figure that DPS are so used to waiting for long queue times, that they accumulate a similar delay in activating the limitbreak. If the boss battle were to last 45 minutes to an hour, than I'm sure we'd see them hit the LB button eventually.

I've gotten used to slamming the LB at less than 1% as tank, just so a DPS can yell at me for wasting it as the boss death animation plays.