>Why is it so easy for me to open the car door

Because this part of your brain has been optimized for hundreds of millions of years. It's been around a long ass time and takes an amazingly low amount of energy to do these things.

On the other hand the 'thinking' part of your brain, that is your higher intelligence is very new to evolution. It's expensive to run. It's problematic when giving birth. It's really slow with things like numbers, heck a tiny calculator and whip your butt in adding.

There's a term for this, but I can't think of it at the moment.