Why? The romans build lots of infrastructure. Roads, sewage, irrigation, .. and they used the materials and technics of their time.
Can't we really do better today, with all our advanced technology?
Or can't we, because everything has to be cheap, cheap, cheap?
I mean, the romans used slaves. That is cheap. But we have machines.
Do you know the movements of the supply and demand curves of the materials required since Roman times?
These types of hypotheticals are a waste of time, and only serve to illustrate hubris, as if something as complicated as comparing Roman construction and resources to modern day construction and resources could be possible.
Well, one can compare the results.
Personally I like things that are build solid and can last a 1000 years.
But I don't think I said the romans build better. They just had a different intention: long lasting.
You implied a whole host of things with this statement, and considering the topic of this thread, also ramming a Roman bridge with a fully loaded modern day cargo ship.
The point is there are so many moving parameters, it is nonsensical to take 1 result of 1 technique from 1 point of time and use that as a basis for what one can expect at other points in time.
"If we decided to build like Romans did, there would likely be very little infrastructure"
Why not? They did not have the intention to make the bridge resistant to such heavy loads, as that was not a use case at that time.
They had the intention to make a bridge to last as long as possible for the traffic at their time. And they surely succeded with this.
Now whether they also could have made a bridge that resists 18 wheelers for 1000 years, well, I don't know.
Also we know a bit more about the romans than just their bridges.
Which is to say "let's just pretend that all of those pesky details do not exist".