Buried Cities

So when archaeologists find ancient cities, they set about digging, in order to unearth them.

Which raises the question: how did the get 'earthed' in the first place? I mean, did the ground open up and swallow them whole? Or did they just slowly sink down?

I've heard it's that dirt and 'stuff' accumulates around them over the years, but certainly that would mean ancient cities would be recognisable as city-sized mounds, right?

Or more to the point: in California, there are forests filled with trees that are thousands of years old. Why haven't they succumbed to years of blowing dirt? Certainly, no-one's running around forests, brushing away the dirt.

And... the biggest problem I have with all this: how are ancient buidings discovered while, like, digging subway tunnels in continuously-inhabited ancient cities, places like Rome or Damascus? This implies that a certain building, presumably an abanoned one, slowly got covered by layers of silt, over a period of a few hundred years (I guess that's how long it takes...), and all the while the neighbours never bothered to say, 'shouldn't we do something about that half-buried building'? I mean, there must have been some prime real estate in Rome back in the day. Why did no one say, 'Let's sweep that dirt away and start using that building again'?