Because either a coach is waiting for the last one off downstairs with everyone getting impatient, or the groundstaff want the last disembarker off quickly so the gate can be locked and the cleaners enter to get on with the tight turnaround, that's why! Don't be offended- there is just no time, at any time! What do people think we do on turnarounds? Usually late, we slide seats back and chew the cud? Shutdown checks to do, get new paperwork, decide fuel, run out to get fueller who suddenly seems unable to flick the wing switches, rush walkaround, load FMS, tech log, brief, checks, set up plane, go. No time for even a cup of tea! However, if only to apologise to an important passenger (other crew, ops staff, ATC), it is nice to know they are there.