Well, here's a few explanations, though they may not be correct: Eins.) The sentence, "The coworkers don't work," sounds more, um, harsh than, "The coworkers don't do any work," because your sentence could be more flexible in its meaning; it sounds like you're saying they do little-to-none, instead of none at all. Although, being honest, they are extremely similar. Zwei.) Okay I'm already out of explanations, I was just trying to SOUND smart, but hey, Duolingo probably accepts that now so I really don't understand why I'm trying lol. At least I tried :) My apologies for wasting your time.
It's probably just that "work colleagues" it's not in die Eule's list of translations for Mitarbeiter. But to be honest, "co-workers" is possibly not the best translation either. Mitarbeiter is more "employee" or "staff" than co-worker, which is probably better translated as Kollege.
Notice the Die and the arbeit
en. The plural form of Mietarbeiter is the same as the singular form, so one must look to the inflection of the article (die, which is either feminine or plural) or, more importantly, the verb's conjugation (which, with the "-en" suffix, is for a plural subject).