We used to get a brand of bagels here that were fully pre-sliced because they were in the same style of bag as a loaf of bread. So being fully sliced wasn't a problem, they weren't gonna shuffle around in that bag. Not sure why they aren't all like that. Also despite being fully sliced, they did still stick together a little bit while fresh. But it was only their humidity holding them together, once they got a bit stale the halves fell apart from eachother.

But yeah, not sure why that is so rare, I can't think of a downside to it that other manufacturers are trying to avoid. Maybe different types of bagels need different solutions? The ones we used to get were cinnamon raisin.