The Yucatán peninsula is remarkably flat. I've walked to the top of Ek Balam, and you can just see little of green all around the surrounding landscape; I suspect a number of them are uncleared (not undiscovered) Mayan structures. The forest is quite dense, but these are fairly obvious.

They are often left undisturbrd for a number of reasons, including that archeological techniques are fundamentally destructive. Leaving sites for future people to explore also leaves intact knowledge that future technologies may be better able to uncover.