It's actually quite a lot more than a bit more secure. Think of it as the difference between traversing no-mans land on a bicycle (current systems) vs in an armored personnel carrier with armor thick enough to stop EVERYTHING that the other side can throw at it. Sure, you could just open the door of the APC and take a hit, but if you don't do that, you're pretty safe.

I do not understand why these two things make their users so enthusiastic

Are you trying to think logically about that question? Because that would be a huge mistake.

flatpak, which is something which would really get to my nerves.

Yeah, not my favorite either. A BIG part of why I'm not loving the majority of the immutable systems I have investigated is because they all seem to heavily lean on tech like flatpak, snaps, etc. and there are valid logical reasons for that, but I'd really rather not go there...