In “who left their bag here”'s case, the gender of the person is not known because their identity isn’t known, so it doesn’t feel strange to use (for us old farts). It can take some effort to retrain your brain to use “they” when it’s a single person whose identity is known (speaking from experience as a Gen Xer).

Or the person might just be a transphobic asshole. But I like to think that most just need to educate themselves on using pronouns correctly.