Most of them did collapse and fail or were otherwise scavenged for materials and lowered a few floors. So it’s not like these were all lasting for some massively absurd timescales on average, what we have are the well built ones. We probably do have plenty of structures that will be around in a thousand years with proper maintenance, it’s just that most large scale building of comparable sizes are only about 200 years old at most, which is roughly comparable to when the larger Roman building in Gaul and Britain started to get a bit rough according to chroniclers.