The Colluseum and Flak towers are still around because the amount of concrete used was collosal (sorry for the pun :p)
Same with Hoover Dam which is still curing
Most of our modrn structures use the steel and concrete as the skeleton and fill in lighter materials with the rest . Once the skeleton buckles it doesn't take long to bring it down. Look at the towers collapsing on Sept 11. All the conspiracy nuts talking about how jet fuel won't melt steel, it does produce enough heat to weaken steel. When one floor collapses it's weight collapses down to the next, which collapses the weight of two floors down and Cascades down to the bottom