First, architecture followed the same trend as modern art where taste and beauty have given way to being original and conceptual. This results in very mediocre constructions. Paris attracts millions of tourists who come to admire its palaces and tasteful haussmanian style. No such enthusiasm for the modern buildings of Frankfurt. Not to mention the concrete-made brutalist horrors and other modern architectural “creations”.
Second, apart from the said brutalist bunkers, all modern constructions are designed for a very limited life (even infrastructures like bridges). I was visiting the Pantheon in Rome last week, a 2000 years old building which interior made of marble and stone looks almost new (after many renovations I am sure). What will be left of our glass and steel buildings? They are merely more durable than a mongol tent. The only trace historians will have of our civilisation is the selfies we publish on Facebook really.