That’s not an American thing that’s a movie/TV making 101 thing, it’s for lighting reasons at night, I guess it’s this person’s first day watching movies.
I was literally in a movie recently with inexplicably wet streets, and the director said it was because the extra reflected light looks great on film.
Those aren’t film, they’re tape. Film is what’s in the camera or shown through a projector and are visible to the naked eye. VHS, Betamax, and video2000 are magnetic tape formats that aren’t viewable with the naked eye. Regardless, “on film” is still a universally acceptable term for “on the recorded video” no matter the format because terms stick around in industries
Continuity. You can’t make wet streets dry, but you can make dry streets wet.
Plus it makes for nice reflections.
Water fall from sky
The streets are wet so it’s easier to perform stunt driving.