They can arch the bridges, they can build them out of wood-then-metal, they can anchor them deep in the plateuas, they can augment stone bridges with metal struts, there are many ways they can get around this.
And as for the car analogy, it seems more like having a car that runs, but it isn't fast enough to win many races, and it breaks down easily. It would make a lot of sense to get a new car, one that's faster and more reliable, even if it is more expensive.
And going back to what Vanstorm pointed out, the merchant got the scrap metal cheap because it was easier and cheaper to build things out of other materials to Soulcast later than it is to melt the scraps down and build them the regular way. It sounds like Soulcasting is fairly economical to me.
Munin:
Also, the concrete buildings withstanding plane crashes... you misunderstand. It left a plane-shaped hole in the building at the top, but the building didn't collapse. Stick a boulder-sized hole in a stone bridge, and it'll fall apart.
Except that the boulders don't leave boulder-sized holes in the barracks. They can build these bridges as strong and as supported as they need to. They're not limited to conventional building techniques. Maybe Soulcasting is slow, maybe it's hard to do. But we've seen no indication that they've even tried.