Boyfriends dad never closes the bathroom door while he's in there!!

I've accidentally walked in on him a few times because I haven't heard anything before hand and it's so annoying. Why can't he just close and lock the he door?! It's got to the point now where he will go to bathroom without closing the door regardless of who is in the house. I know that everyone goes to the bathroom and it's natural but it's just weird that he never closes the door. His wife has told him about it a few times but he never changes. Just needed to rant as I literally just walked in on him again 😡😦