Why are bridges made of steel as they are good conductors of electricity?

Steel is much cheaper, easier to work under die/press and tooling. Security with aluminum need much more engineering because it's more prone the shear, less tolerance to temperature change.

Also, if daily would be made from a material which not rust and almost no decay, they'll make bankrupt, they won't sell as much they sell today. That is the true reason. They know how to make cars which last over 50 years with the same bodies, engines and drivetrains, but what would be the initial cost vs the long term sales ?

I don't know about why about the bridges, it probably has same kind of reasons.