Honestly the answer is complicated, and I mostly see this asked from people who are dedicated tanks or healers. You notice bad dps cause each party gets 2-4 of them, and as the aforementioned classes you very rarely outside leveling/full party run into another tank or healer.

Now that's out of the way, we can unpack the answer to the question. Dps have no tools to know how much dps they are doing cause SE is more worried about feelings than results. Since dps always come in 2-4 per group, it means the slacking dps may not even know they are bad cause the other 1-3 dps provide the results needed to get through all but the toughest dps checks. It's akin to someone being moved up grades without being taught anything, then slapping them with the unfeeling hand of reality when their professor reads aloud their essay that sounds like a 5th grader wrote it while high on cough syrup to the thunderous laughter of their peers.

The above is enough, but frankly it gets better. Most dps have a chip on their shoulder, placed there by the fact the game makes it crystal clear they are replaceable low value resources. The bonus for a duty is almost never for dps, commendations hardly fall your way, and if a group is awful you often grit your teeth cause 30mins for leaving on top of the 10-30mins you'll wait to get in another one means you wasted most of your playtime for the day.

Now mix those two together and you have the witch's brew that answers your question.