Because sound travels faster and is more distinct when transmitted through solids compared to gases. This happens because solid particles are closer to each other than particles in gases. So the sound of enemies moving will be easier to hear when their ears are to the ground.
If you want to try this for yourself, hit a table when you're standing, then put your ear to the table and hit it again. It'll be much louder when your ear is to the table.
Hope this helps!
