Some people like to feel smart, I guess coming up with silly examples of why something they never used won't work make them feel smart. All they have to do is ignore the fact the 99% of their criticism is silly and the remaining 1% has already been solved by professional that have worked on the thing for years.
I think it's because some people, and I include myself in this, have to deal in the real world, with real people as part of a real job.
I need to totally think thru potential issues and try my best to come up with fix's that will stop errors occurring.
Same with programming a game.
What happens when someone enters an area in the game or does something they are not supposed to do.
You can't have the game crash, you need to find and fix such things.
So when something is presented to people who HAVE to think like this, the instant reaction is to think of the potential weak links, and wish to see them tested.
Just "doing as you are supposed to" is pointless, you need to test out all the things you are not supposed to, and see if the system? still works, or does not fail in some way.
Even with a road bridge.
You don't make it, so it's fine with a expected load in nice weather.
You need to design it, so it takes a crazy loading, and the worst weather x 10 you can ever think of.
THEN it's safe.