Models. All those glorious buildings and they made flash cards of models. Either the real thing doesn't exist yet, or the photographers wanted too much money.

The Ford exhibit was a big hit of the '39-40 Fair, and no doubt lots of people wondered how they'd top it. Wikipedia:

Ford Motor Company presented "Ford's Magic Skyway", a WED Imagineering designed pavilion, which was the second most popular exhibit at the fair. It featured 50 actual (motorless) convertible Ford vehicles, including Ford Mustangs, in an early prototype of what would become the PeopleMover ride system. Audience members entered the vehicles on a main platform as they moved slowly along the track. The ride moved the audience through scenes featuring life-sized audio-animatronic dinosaurs and cavemen.

(The video was taken from this YouTube page; no attribution. I color -corrected it for your convenience.)