Many were frozen in time, packed under tons of sediment, ready to meet with the pick and chisel of a human who 65 million years later is searching for dinosaur bones. A great deal can go wrong in 65 million years. Rivers and ocean shorelines change. Islands grow and disappear. Mammals take over from reptiles.
Civilizations rise and fall.
And Madonna comes out with another album. At that time, huge glacial ice sheets covered more of the Earth's surface than during modern times. Just try rubbing a stone over some hard bedrock over hundreds of thousands of years. Now think about this on a bigger scale. Not only do you get pulverized stone, but like a giant sander, the glacial retreats and advances wore down rock layers.