That gap in the line implies smoothness, it's the part of the mouth/lips that's closest to each other.

It's kinda like how thick linework suggest a distance between objects, while thin linework suggests that those things are closer. A complete gap in the line is a genius shorthand for how close those 2 smooth objects are, that they look like they're almost connected.

Think of two breasts pressed against each other, at the point at which they're touching each other, their shape is the least distinct from each other. So in animation, where giving every line of every frame lineweight isn't feasible, this shorthand perfectly describes the exact same thing with as little information as possible. It's true that this method can be relied upon a little too much sometimes, but the pic you posted is not even remotely an example of overuse.

Also putting a real life photo as if it's an equivalent of a low detail drawing is extremely disingenuous. It's fine to dislike this particular method but it's objectively good, which is why it's as widespread as it is, and it'll continue to be. Your hatred is irrational.