Some, not all. Some believe it is a ball, but don't mock those who believe it is flat.
I, for one, don't think we should make it a practice of coddling absolutely absurd, unfounded, beliefs...like a flat earth. That's how you dumb down a population.
In terms of being "open minded" vs. "closed minded", there's a difference between instantly rejecting something that hasn't been studied at all yet out of bias (for instance, if a medical researcher proposed a theory for cancer treatment and someone rejected it before even listening to the details)...that's truly being closed minded, ...and rejecting something that already been definitively disproved numerous times through a wide variety of modalities. "Flat Earth Theory" would fall into the latter.
The fact that they have to resort to the traditional conspiracy theorist method of debate (dismissing all evidence as "tampered with" and "in on the hoax") is quite telling. "Those picture are photoshopped, NASA tampered with the data and made stuff up", etc...
I also don't think it's coincidence that people who believe in flat earth also happen to be professors of just about every other pop-conspiracy theory out there. (9/11, JFK, Moon Landing, Vaccines, etc...).
I think some people just have a mental predisposition to want to believe the "alternative" to the conventional... I'm convinced that if the earth actually was flat, and flat earth was what was being taught in mainstream science, those same folks would have a round-earthers group simply because they always want to oppose the mainstream at every chance they get.