Or could it perhaps just be that the requirements of the bridges are different? Things like oscillation tolerance, minimum width etc. Perhaps dictated by local laws and/or the kind of target public they want to attract, the average number of days a year the weather would permit transit etc etc ?
You are pretty much exactly describing how corruption works in a well regulated country. Since officials can't just pick the contractor but have to take the cheapest bid for the given requirements, they have to doctor the requirements until the contractor they want to hire has the best chance of making the best bid (or be the only contractor that effectively make a bid at all). This is exactly how it works from the lowest up to the highest level of government.
Since actual corruption is so hard to measure, we rank countries along a line of "corruption perception", i.e. how confident are people. Germany has an extremely good reputation. I believe you have to look at individual projects though and use a first principles approach: how much should it cost? What level of incompetence is believable vs plain corruption? Does the project make sense given larger government policy in the first place? Airport Berlin-Brandenburg, Wirecard, Nordstream are just the larger known cases, where corruption is the only possible explanation, there are tons of smaller projects as well, on which nobody bothers to put the spotlight.
It sounds like you are describing monopolies or regulatory capture. And sure enough, the US and Germany have some of the least competitive infrastructure construction markets.
