It's not as difficult as you might think. Or, more precisely, it's a lot more difficult to destroy a bridge than you might expect. Look at how much trouble Ukraine has had trying to keep the Kerch Strait bridge out of commission. Or look at the hundreds of bombing runs the US did in Vietnam to try and knock out one stupid bridge.
It's easy enough to temporarily shut a bridge down. Bombing its road deck is pretty easy. The deck is usually exposed to the sky, so bombs can be dropped right on it from above. And the deck is thin enough to be fairly vulnerable to bombing.
The problem is that bombing a bridge deck doesn't actually destroy the bridge. At best you create a hole in the bridge deck a meter or two across. That can shut a bridge down temporarily, but it doesn't actually damage the bridge's structure. To repair the deck you do something like:
- Clean up the hole. Remove any loose concrete chunks with a hammer, cut off any exposed rebar with an angle grinder.
- Drill holes at regular intervals through the edge of the hole.
- Run new rebar spanning the hole. Epoxy rebar into old bridge deck.
- Attach some plywood to the bottom side of the bridge deck to server as as mold.
- Pour new concrete and let cure.
This can be done in a day or two. And on a multi-lane-bridge, traffic can continue to flow around the bomb site at reduced capacity.
If you actually want to destroy a bridge, you can't just hit a random spot on the bridge deck. If it's a big truss bridge, a direct hit on a key truss member could cause the entire bridge to collapse. But those are very precise targets to hit. And if it's just a bridge deck sitting on beams sitting on piers, the only way to destroy the bridge will be to knock out the support piers.
The problem with destroying them is that bridge piers are both hard to hit and extremely resilient. They're protected from above by the bridge deck, so to hit them you'll have to glide a bomb in horizontally. And even if you manage to hit them, they're utterly massive. They have to support the entire weight of the bridge above. Plus they're often directly in a river's channel. This means they're built to survive the forces of water current and ice collision if the bridge is in a temperate climate. Plus they're built extra thick to resist the corrosive effects of the river water. Now, if you do actually manage to knock out a support pier, replacing it will be a nightmare. But that's a very difficult thing to do.
It's actually a lot more difficult to destroy a bridge than people realize. Knocking a hole in a bridge deck is easy. But bridge decks are easily and quickly repairable.