They do that anyway. The evaluation process in the contract has to deal with that.

I don't know, it just seems to me that having two engineers devoted to find ways that you can save money, whom, presumably also get rewarded by how much they save (this could just be a nice speech by someone but I think if you saved significant amounts maybe a bonus?) would inevitably lead to some situations where they realize that the grade of cement or something similar had not been specified for something and it probably means that, barring significant seismic events in an area that has a fault that has not done anything significant for a century, the thing being built would not need any repair for 45 years instead of 65.

Obviously this example is not a real world one as construction is not my area of expertise but I do think I have insights into mundane human mendacity. Then again I am also prone to cynicism.

In the US, on large structural projects, things are generally well-specified, either by standard code, or long before the project goes to bid. The owner (usually another firm representing the government that's paying for it) and the contractor inspect and measure all the work done each day. There are undoubtedly going to be mistakes and corners cut, but that's what safety factors are for and it's hard for problems like that to be endemic without some serious corruption.

On a related note to using the wrong concrete, things like this can be an opportunity to see master craftspeople in their element. We had a truck of concrete show up and the foreman rejected it with almost no hesitation. "Why the f*&($ won't you sign on off this!?" the driver yelled. "It's smells wrong", said the foreman. Nobody else noticed anything amiss, but he was the boss, so the truck went back to the plant and sure enough, they'd mixed it wrong.

This is an interesting point. There have been a number of construction-related scandals in the last thirty years of building public housing estates in Hongkong. Either the contractor used weak cement, insufficient rebar, or didn't drive piles deep enough. The fact that they were not caught until the building was completed is terrible. It says the QA by gov't was seriously flawed. (It has since improved a lot!)

I think the offset is to have a mixture of QA/checkers and gov't regulators who are onsite checking things. With huge civil engineering projects, sometimes the gov't will hire a competing civil engineering firm just to be onsite checking the work. That balance probably works.