If you're getting ready for work in the morning and you're suddenly hit with an overwhelming panic attack and you feel afraid to leave your house, that doesn't leave much time to request the day off to use for vacation.

Most people will judge you if you call in and say "I am experiencing extreme anxiety this morning and I'm afraid of leaving the house, so I won't be coming in today." So instead you shoot your coworkers/boss an email or a text that says "Hey, I'm not feeling well today. I don't want to get you guys sick!" Because that makes you seem caring and thoughtful instead of a weak coward who can't handle the stress of real life.

Even people with "serious problems" have to pay the bills.

*Edit: I just realized that when you said there might be a more serious problem you might have meant that the employer is not offering enough vacation days. If that is what you meant, then I'm sorry for unfairly maligning you with my last line.