What's up with this film trope?

Doesn't bother me in the least.

It always seems odd to me on TV shows when someone just walks in without knocking. On Seinfeld it was part of the joke with Kramer. But on many shows they just treat it as regular behavior.

I haven't locked my doors or taken the keys out of the ignition in years. Serious middle-of-nowhere country living.
However - if someone is foolish enough to walk in my house unannounced, they're likely to be shot.

It makes more sense when it's a cop or detective doing it, rather than a regular civilian. More than once, I've seen a cop/detective in a movie/tv show, knock on the door of a home to someone who might be a witness to a crime, or knows the murder victim, they find the door unlocked and partially open, come in, and find some kind of disaster. Sometimes the person is also dead with blood everywhere, they might still be alive and badly injured, possibly by the same person who did the murder, or they're not home, and the place has been ransacked, and the person in question might have been kidnapped after a big fight, leaving a mess. Just depends on the cop story.