I wonder what the ancient Greeks and Romans thought about the buildings they were building? Did they think that if it was still standing in 150 years they'd be happy? Would they be surprised the buildings were still standing 1000s of years later, or was that what they were hoping for at the outset?
The thing is, back then statics and architectural design were in their children shoes still. So if your roof was 3 tons, your supports had to hold up 300 tons because you had no better way. Nowadays, it holds 15 tons for 3 tons + 12 tons snow, for example, or in more äquatorial areas, it's 3 tons + 1 ton rain water.
It's not hard to make things last when you helplessly overspec because you have no idea how close you are to collapsing and the material quality is extremely unreliable.
