I'm glad you added the second half, I immediately started replying after reading the first sentence that the worst part about shaving in movies is when they wipe off the rest of the shaving cream without finishing shaving, hahah.

Absolutely Psychotic.

Similarly, toothbrushing.

This silliness exists because filming actors is hard. Like how almost nobody eats, in movies. It is a serious and committed choice for an actor to genuinely consume food... because they're gonna have to do it a dozen times, whilst delivering dialog, and hope nobody (including them) screws up or half-asses it.

And when they do show an actor eating, it's usually to convey a particular tone. Like Denethor. So: why is the actor shaving, in the first place? Generally the answer is not an answer that requires putting the entire process onscreen. So there's this shorthand that's not so much unnoticed as accepted. It's as much a part of the fiction as the fourth wall. Audiences don't question how everyone speaks clearly all the time, or how actions occur perfectly framed to a nonexistent camera. We just get the idea that someone is shaving, and ideally derive some characterization from the details of the scene, without having to watch an actor pantomime away every last drop of foam with a shiny squeegee.

You'll notice a lot more like this in foreign films because you're not inculcated to their tropes. There's some jump scares in Japanese horror that become kind of silly when the ghost is literally just wearing a handkerchief over their face. It serves the same purpose as Jacob Marley showing up in a tattered suit.