Modern concrete could last as long ad Roman concrete, if not for the fact that we use rebar which inevitability corrodes and is the limiting factor in how long

Modern concrete could last as long ad Roman concrete, if not for the fact that we use rebar which inevitability corrodes and is the limiting factor in how long those structures can last. But that same rebar allows us to create structures that the Romans never could have. Concrete structures with huge spans and thin walls. It's not that we don't know how to build things which last long. Rather, we choose not to because building that way has tradeoffs we usually don't want to make. This is also the reason we make things out of concrete and rarely out of meticulously stacked rocks.