system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why is it so hard for some people to rub their stomach and pat their head at the same time?

I know there are many variation of this, but this is the most common and well known, and I would assume that the answer to this would work for the others.
gpt
It's a limb independence issue. Your limbs want to work in tandem with each other to do tasks (like walking), so there is some extra training needed to get them to do different things. Most of these things you pick up as a toddler and never notice that they were difficult for you. A good example of something that adults struggle with is drumming. While there are certainly people that can pick it up quickly, a lot of people have trouble keeping the arms at different beats while using the kick drum at another. That is of course a task very high on the limb independent scale, but hopefully it'll help you understand.