I think those timelines are massively over-exagerrated, there are huge gaps in the timeline but your point still stands. The current architecture mirrors the current rulers and their society 💯. Ugly concrete and simplicity everywhere. No beauty to be found at all.
Discussion
Evidently not all eras were magnificent, after the fall of the Roman Empire there was an important vacuum and even involution.
Things like water canalizations with lead pipes, sewage, cement and several techniques were lost for centuries.