The concrete dome of the Pantheon in Rome remains stable enough for visitors to walk beneath, and some Roman harbours have underwater concrete elements that have not been repaired for two millennia – even though they are in regions often shaken by earthquakes.
Whence this remarkable resilience of Roman concrete architecture? It’s all down to the chemistry.
I have heard too there are differences in available raw materials. Even our newer concrete is not as good as older.
Notice also they said common cement too. I suspect supply and demand meaning cost and obsolecense are what we design for. For that matter too cheap patio blocks are not as good as expensive ones. Sad but we do not build for even decades let alone centuries.
Keep in mind too that technology does not automatically improve. For tech to even continue at the same level we have to continually practice it.
Our newer concrete is better for the goals we are setting in construction, which does not tend to include permanence. Our goals are mostly about strength to weight ratios and other properties that allow for massive numbers of floors with and as little mass as possible with reliability measured in decades. Basically guaranteed to last reliably with minimal upkeep.
We are continuously practicing new ways to build with concrete, wtf are you talking about?