Why are ancient ruins underground?

Not all of them are. Plenty of ancient ruins are above ground. That said, the simply answer is detritus builds up. In old forests you are walking on layers and layers of decomposing leaves. Every year new layers of leaves, wood, mulch and other detritus are added while at the same timethe lower layers slowly decompose into compost and eventually earth. At the same time new life grows on top. In some forests that layer can be meters thick. It's not living material that constantly adds to the surface of the earth. Over the millenia, erosion can wear tall sharp mountain peaks into soft rounded cliffs. That's tons and tons of hard rock worn down into sand to be spread on the wind. In some places like the sahara, structures can disappear under the wind born sand in a matter of months if not cared for. Eventually all is rendered to dust on the wind. That which is not worn down will be buried long before that.