How can just sedimentation of sand, bury big building to such depths?

If lack of habitation by humans eventually results in burying of things, then what about the places like Chernobyl, which has not been inhabited for a long time?

I am not a physics grad or anything, just a curious guy.

Any help is much appreciated.

Since you mentioned gravity in your question I'll breifly introduce another curious component (there are others,see reference below) which results in globally non-uniform sea level changes. We are not experiencing a lower level of gravity (gravity is a constant). The amount of gravity that something possesses is proportional to its mass and distance between it and another object. On Earth mass is obviously not evenly distributed, with each location on the planet experiencing a different mass-dependant perturbation of the gravitational field. So for example, as an ice sheet forms and grows mass increases which results in a gravitational perturbation which attracts the ocen toawrds the ice sheet but as the ice sheet melts and the gravitational perturbation reduces that water that was previously drawn towards the ice mass is redistributed in distant locations from the ice mass. This (counterintuitively) means that the greatest rates of sea level rise as a consequence of Greenland melt are actually experienced in the Southern Hemisphere.

Hope this serves as a useful introduction. I would usually include more references but I'm writing this on my phone right now whilst travelling on a train. One key reference,which is written in a non-technical style, worth checking out is : Sea level is not level : The case for a new approach to predicting UK sea-level rise. / Gehrels, Willem Roland; Long, A.J. In: Geography Compass, Vol. 93, No. 1, 01.03.2008, p. 11-16.