Modern builders are operating on a much different set of constraints than the Romans. Simply include 2 additional factors in the equation (cost and harsh climate) and the Romans would not have fared nearly as well. The circumstances under which all of these ancient structures were built were more-or-less ideal for longevity. Is this a coincidence? No, it's survivorship bias[0].
How is the climate worse nowdays than in the past 2000 years? For a sensible comparison, look at the (many) wars these buildings had to endure as well. It's easy to claim survivor bias, but the durability of these buildings is simply a matter of technical analysis and specification, as with modern construction. I had to choose the material for a building facade (renovation) recently and the best option for longevity was: natural stone, the worst being the most commonly used type.
When I mention survivorship bias, I do so in order to highlight that people are being selective when talking about those buildings which have survived and ignoring the vast multitudes of Roman and other buildings which have not.
As for climate? Yes, the climate where I live (Ontario, Canada) is vastly more harsh than Rome (ancient or modern). Why do I bring this up? Because generally the people making these sorts of survivorship biased observations are also neglecting to control for other significant variables such as location.
You also didn't mention cost. Many of the great Roman structures which have survived to this day would have astronomical costs should we attempt to build them in the modern day using ancient building techniques and materials.
> You also didn't mention cost. Many of the great Roman structures which have survived to this day would have astronomical costs should we attempt to build them in the modern day using ancient building techniques and materials.
I'm not so convinced as far as materials are concerned. Techniques have improved and modern ones could be used (cranes etc.). The main issue I see here is that materials aren't even available anymore, nor are they legal w.r.t. building codes. So even if cost wasn't a factor, I don't see how anyone could build a modern building with Roman concrete, marble etc.
