The article states "smooth layer of fine white limestone on the outside" and that the current outer rough surface is due to "hundreds of years of pollution and weathering".
"In AD 1303, a massive earthquake loosened many of the outer casing stones, which in 1356 were carted away by Bahri Sultan An-Nasir Nasir-ad-Din al-Hasan to build mosques and fortresses in nearby Cairo."
It seems like earthquakes did for most of the other "seven wonders of the ancient world", too. The others far more completely than the great pyramid, though.
It only states that it's "colored a dark sandy brown from hundreds of years of pollution and weathering". Maybe I'm being too literal, but I don't interpret this to mean that weathering and pollution are the only causes of all of its changes in the past 4500 years.
It was my understanding that the outer surface was polished marble, and that it was removed by looters, not natural degradation. But I haven't been able to find a good source, aside from https://www.reddit.com/r/history/comments/cxgj96/what_happen...