How is leaving the door open a big deal? I just leave it open and pretty much only close it if it's been open for more than 24 hours or I need to use the showe

How is leaving the door open a big deal? I just leave it open and pretty much only close it if it's been open for more than 24 hours or I need to use the shower (it's a small bathroom). No mildew or strange smells. I think the problem is that people these days don't want to deal with even a minimal amount of maintenance.