Yeah, let's reuse steel that has been weakened by unknown amounts and rebuild the bridge with the exact same flaws that caused it to be vulnerable to this type of collision.
This is major RTS gamer brain. In those games, the scarce resources are the raw materials, because it's more dramatic to send your harvesters into the unknown to be attacked by enemies, while the actual knowhow on how to build stuff is passively "researched". So if you need to build a bridge, you magically have the planners and the engineeers and the building crew ready and waiting, but they need steel and stone to start, which you never have, because you just panic-built a bunker to defend your base.
In the real world, getting enough steel to build a bridge is easy enough - I imagine the lead time to be a couple months. Actually finding a team able to plan and construct one is another matter.
Judging from how much steel they'd have around at a single warehouse near to where I used to live, I'd hazard to guess that it would take much less time than a couple of months to get the steel.
But yeah, it's a moot point, because like you say it's going take far more time to design a new bridge and get together the crews to build it. We don't have some idle SCVs around just waiting to start building as soon as you click on the the location.
And I'm no engineer, but I'm guessing the steel from a bridge that collapsed would be too warped to be useful and the pieces that looked ok would have to undergo testing to make sure the stress they went under didn't compromise the material in some way. I'd guess it would be more expensive to use that steel than to just use new steel.