pretty sure your parents are fuckin and that is why the door is locked.
really who locks their door at home unless they are jackin it or something similar.
i lock my door all the time because for years my brother would just WALK RIGHT IN without knocking even if the door was closed, sometimes even if i was asleep (he'd wake me up at 2 am for important questions like 'hey do you have a sharpie i need one') and so i started locking it to keep him out, now i just do it out of habit
but i am kind of in the habit of locking all doors because i lock the main door to my house even if i know that someone else is still outside just because i do it automatically, it is a good habit most of the time but sometimes it can be a problem (i.e. locking someone out and having to come back downstairs to let them in)
the only door i don't do this too is my dorm door, because i'm more obsessed with checking to make sure it's UNLOCKED when i leave for a few minutes, i hate locking myself out and i don't want to carry keys to walk two steps to the bathroom so i just double check it to see if it's locked or not every time i leave
edit: fuck i hate when i don't realize how long a post is and then i post it only to find out i wrote three paragraphs on my door locking habits, it looks ridiculous