Having a part-time job can really take it out of you. Full-time jobs might technically take up more of your time, but they're usually much more predictable. If you're working from 9-5 for five days every week, you don't have that added stress of waiting for a schedule to come out or begging your coworkers to cover your shifts. The unpredictability of part-time work adds a lot of stress to a job that shouldn't take over your entire life. Constantly shifting schedules make it much more difficult for part-time workers to find a second job, limiting their ability to make money. Even if they don't want a second job, it makes their life unnecessarily stressful.
One part-time worker posted texts he got from a manager following a surgery. They claimed they wanted him to be available (meaning "on call") for all normal business hours (9-6), which doesn't sound like a part-time gig to me.