I wouldn't say fragile. A bit of Roman stuff is still functional today. The pyramids are holding up too. Cathedrals also have a massive stability streak. The stuff that has been falling over is mostly brick, steel, and concrete. And that's due to lack of required maintenance. I could even use the opposite word to describe them: strong and solid.
And plenty of cathedrals collapsed without proper maintenance, or were destroyed by invaders, or had their stones taken by locals for use in building. All it took was a spark in the wrong place to destroy Notre Dame.
Not if you actually look. Romans invented a concrete recipe that was lost for a millennium. Their ports were holding together longer than any modern concrete. This Roman concrete recipe was recently rediscovered and has been getting traction again.
