Women holding kids in movies always jump up and down

Well, maybe not 'jump', but I don't know what to call it.

For some reason, a 'mother type' in a movie can NEVER just 'hold a kid/baby/whatever', but they have to constantly move up and down. Constantly.

Always.

Why is that? That kid is old enough to run around and explore things on his own, but yet this 'mom' just keeps holding him and moving up and down. So if such movement is needed, why not just let the kid move on their own? What's this obsession of 'holding' them always anyway? Where's the independence?

Anyway, I just don't get why they constantly move up and down while also holding their hand to the back of the kid's head. It's like a movie trope you see constantly and never realize to question it until you see it one too many times and start thinking, what the heck?

Why does this happen? Do people do this in real life?

I get it if it's a young baby that you need to burp or something, or you somehow magically can get them to sleep (after they scream for three hours), by doing this weird ritual, but why is it so common that you have a very slim chance of ever seeing a woman just holding a kid on their arms and letting the kid actually look elsewhere besides the woman's shoulder?

I don't get it, it doesn't seem to make any sense.

Yes, sometimes, fine. But ALWAYS? What the heck?