Why do I have to take a shuttle train to my gate in an airport like SEATAC?

I am very familiar with O'hare airport, which is large, definitely bigger and busier than Seattle or Miami - yet in those 2 airports I'm forced to take a shuttle train after I go through security.

Why?

O'hare is larger yet no train needed. I guess there is walking involved, and since I enjoy walking maybe I never noticed a train is needed at O'hare?

I love traveling but FUCKING HATE flying. I hate it with the burning passion of a thousand white hot suns.

So many things do not make any sense. Like how boarding is so damn inefficient, like the security theatre anal swab bullshit we not only have to endure but pay for with taxes. Also, we've been to the moon, we need transmedium passanger craft to go into space, where there is no friction and once in space you can get almost literally anywhere in 45 minutes. Instead of politics, war and reality tv I wish our resources were spent on things like this. But we can't even board a fucking plane the right way.