Survivor bias. Damn near all, like 99.99999999999999% of ancient buildings did not survive time alone, let alone disasters.
If you’re asking how those that did survive did it: Heavy sturdy materials that aren’t fastened. As in gravity holds it together, rather than being designed to withstand gravity. You are just used to our modern way of doing things as cheap as possible with better engineering.
If you want more can you give a specific example of which structure? Each structure usually has its own engineering problems and solutions.
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