The kid was just a baby when cars were invented. He didn’t live through the transition, and probably never saw a cart pulled by an animal. The kid’s parents, as seen in the first panel, did live through the transition from carriage to car, and presumably grew up with the animal driven vehicles.

So, when the kid sees a goat pulling a cart, this is a novel and new thing for him. But to the parents, it’s like going back in time, a relic of the past, so they see no wonder in it, because it’s something they’ve known about all their lives. They thus find the kid’s wonderment puzzling - what is there to marvel at? Nothing new here.

Like the title says, things don’t seem wonderful if you’ve seen them all yoir life.