@Helen744
I wish I knew...
I have a theory that people do a lot of things on autopilot when they are home, such as closing the front door, but that autopilot malfunctions when they are in an unfamiliar setting. Or it might just be 'holiday brain'.
On the other hand, I don't believe that I do these things when I am staying elsewhere, so maybe some people are just careless at home.
I have noticed that it's usually men who leave bathroom lights on. I don't mean every male guest, but I'd say at least half of them, if not more, do this, whereas it's very rare with female guests (both genders are guilty of leaving bedroom or other lights on though).
Whenever I've been hosting a couple and have jokingly mentioned this, the girlfriend/wife almost always exclaims, "Yes! He does that at home ALL the time!"