They do choose to include them. Not always for some greater Purpose, but nothing (more or less) gets in the film without the say-so of, usually, several people.
Exception for location filming without extensive preparation of the location, I suppose, but anything captured there is still included by choice, the location was selected on purpose, shot angles were selected to capture a certain background rather than another one for a reason, et c—even within the confines of natural-light outdoor shooting, there are usually still several options available that would look more-or-less equally good from a lighting perspective, but the one on screen is the one that was selected.
The point isn't that everything means something, but that the entire experience (in like 99.9% of films, anyway—close enough, and covering so much of mainstream cinema, that we may as well say "all of them") is crafted and curated. Not much makes it on the screen unless it's on purpose, barring mistakes like a boom mic dropping into frame or nobody noticing a Starbucks cup on a table in a Star Wars shoot. The character doesn't say something because they just happened to say it, but because one or more creative people crafted that statement (even if it's improv! That's still not actually the character coming up with it, because the character doesn't exist! And on top of it, the film doesn't necessarily include every improv'd line, so they're still curated).
Out of all the shirts in the world, someone selected that shirt for the character—they thought about it, and they picked it, probably at least for some combo of simple visual effect and fitting one or more people's idea of what the character might wear on this particular day. That sort of thing.
This means that it's rarely outright nonsensical to ask why something's in a film. That doesn't mean there's always some deep answer behind it, or that it's knowable or something that can reasonably be deduced or productively guessed-at from available evidence, but it means the question's always relevant.