As to why it might erode faster... I don't see erosion that couldn't have happened in 4500 years. As Harte pointed out we can't know what the weather in that time was accurately enough. These particular constructions date to Twice the age of Rome. These ruins were the same age to the Romans that the Romans are to us.
Also whos to say they did not create some of this apparent water damage during the time of contruction? Supposedly there was water all over the plateau for almost a century? What is the plateau's dranage pattern? I've heard that flash floods can tear through the Giza area and create quite a lot of damage. That would lead to the idea that drainage tends toward the river.