If bridges are earthquake-proof, then why do they send cars airborne during an earthquake?

[BigJimsWornOutTires]

Ah, yes, there I was, headed to a tedious place I go every day. As I moved along with the heavy congestion and entered the Sky Road zone, I saw what was about to happen. I knew without a doubt, "This motherfucker's about to go roller coaster." The road became a violent snake, sending vehicles flying into the air. And just as I joined the other travelers on the unscheduled flight, I woke up.

The very first thought I had was, "If bridges are earthquake proof, then why do they send cars airborne during an earthquake?"