Edit- After reading most of the comments the general consensus is listed below:
1. The unique composite matrix of the concrete used gives it a self-healing property. When cracks form in the concrete, it will naturally seal them.
2. The Pantheon was a very significant structure which led to meticulous maintenance and restorations
3. The Romans didn’t have modern engineering. So they didn’t know exactly how strong they’d have to build the Pantheon to make it last. Their solution was to overbuild the hell out of it.
4. Survivorship bias. There were thousands of buildings constructed by the Romans but very few remain which are the ones we marvel at.
Meticulous mixing, adding of volcanic ash, limestone pieces, and heated when mixed.
There was a paper recently published that talked about it in much better detail. Aparently the limestone pieces in the mix would cause cracks to form there and with the later addition of water would cause a reaction to seal up the cracks.
Anyone who says things like the ancients had “better materials” is just dumbing things down.
Is Roman concrete pretty good? Yea. Is it a magical building material? No. Can modern concrete match up to it? Yes.
The real thing is utilization, and maintenance.
For one. The pantheon only has people walking around inside of it. That is SIGNIFICANTLY less load than say, a bridge with semi trucks driving over it constantly. So it’s going to experience much fewer issues than the modern road.
And, it isn’t perfect, hell there was a really big restoration of the pantheon less than 100 years ago that fixed some pretty big cracks. But that’s the thing. For the entire existence of the pantheon, it was a Roman Temple built by emperors. So they had the money and motivation to take really good care of it. And then AFTER that, it became a church. And in that time the Catholic Church had the money and Motivation to take really good care of it.
Any modern structure that is crumbling is not crumbling because we are bad at building things. It is crumbling because no one has the money or motivation to fix it.
If we really wanted to, we could keep ANY building standing for 5,000 years. It would just take the time and effort of keeping it in good shape.
To add to other people’s answers, one thing to note is that Pantheon was a building of special significance, which means it got special attention. There are thousands of structures from that have crumbled a long time ago.
As one of my old engineering professors liked to say—“Anyone can design a bridge that doesn’t fall down. It takes an engineer to build a bridge that BARELY doesn’t fall down.”
The Greeks and Romans over-engineered. They built buildings that lasted 2,000 years longer than the clientele they were supposed to serve.
We could build structures that last that long today, with ease. We choose not to spend exorbitant money and time on creating something that might last 1000% longer than the desired design life.