Many dinosaur bones have been found. why do we have little information about other parts of dinosaur's bodies?

To date, approximately 1,200 dinosaur skeletons have been discovered.

Scientists can only guesstimate about the other parts of the bodies of dinosaur's because a lot of times, partial skeletons have been found. Or even, when a full skeleton has been found, the skin and organs and everything else is gone. This leaves the scientists to make educated guesses about how they stood, what they looked like and everything else in between based on the skeletons they've found.