OK there's more afoot here. Some of it is absolutely the "CAD/FEA/etc model said that, must be true". Same sort of nonsense as some of the highly touted covid "prediction models" which on close examination after they finally revealed their code after great pressure, turn out to be heaps of random guesses. On this kind of pile of cards of assumptions and lies where the key health plans of major nation-states made. Same story over and over.
BUT there is more at work....
As time as gone by, density of competitive space has gone up, and a richer public demands more function from things like bridges. By density of competitive space I mean things like it's not greenfield location any more, you have to buy land at great cost, remove the existing structure at great cost. Higher demands are things like your new bridge has to be earthquake and hurricane proof, has to not kill all the fish or the whales, has to support mass-transit, and pedestrians, and cyclists. And have some breakdown space so a simple blown fanbelt doesn't cause a miles long backup.
In short, nobody will accept the same bridge as before, a bridge with much higher function is demanded.
It is (for good reason) outright illegal to mass produce and sell a car with 1960's safety levels....
BUT WAIT THERE'S MORE:
For political, commercial, and various social reasons, most schedules are "this is the first date you cannot prove it won't be done" and many budgets are in reality "nobody will believe us if we say it will cost less than this."
Projects are "sold" to the public, to senior management, to investors, with these "nobody can prove it will take longer or cost more than this" numbers.