If bridges had scope creep the way software does, every bridge would start out 1 lane each way and be 6 lanes double decker in the middle before all lanes fly in different directions towards different cities, much less the other side of the river.
I hear what you're saying about using tried-and-true methods. But nothing else has scope creep like software does, because basically nothing else is design-only. That's why software is so crazy. It's ALL design. There's no manufacturing at any point. You can manufacture infinite copies for almost free, i.e. compile and copy and install.
If you could go from bridge design in CAD to actual working bridge in meatspace in ~30 seconds and for $0.01 then yeah, there'd be millions of horrible, horrible bridges EVERYWHERE.
The fact that the design of the bridge takes a year or two and a few million compared to the 2-5 (or 10) years and billions to actually manufacture it means that you can design, redesign, and redesign again until you get a design that'll actually work and it barely moves the needle on the total price tag.
But in software, if you redesign and it doubles the amount of time to complete the project, you just doubled the cost at least.
If it were possible to make software better, faster, cheaper by just imposing some discipline, why haven't dozens of companies done so and taken over the world?