I would assume that we don't have the exact blueprints of how those buildings actually and really looked, so trying to rebuild them would have the risk of ending up like the architetonical equivlent of movies about vikings, with helmets with horns that no viking ever wore.
Also I guess that the sensibility to preserve historical landmarks came later in our history, and so people before us got accustomed to see ruins and cherish them as such. By now we have the skills and the knowledge to rebuild stuff and we just do it.
Also, Notre Dame is an actually active church. You just can't say "damn, it burned down, let's keep it this way and let's have all those masses somewhere else".