How do ancient ruins become ruins? Unless literally everybody nearby dies, how do buildings become so neglected that they get entirely buried in dirt?

Over in archaeology-land, they call this 'site formation'. As always, if anyone wants to contribute further on how things get into the ground, please don't let this post stop you! More insight is always welcome.

While we wait for new stuff, OP, "How do cities get buried?" is a reasonably common archaeology question here, with a few previous answers you can read:

u/acousticsontrial has an overview of site formation;
u/kookingpot covers the general process of things getting buried;
and further expands on that answer here;
and u/SleepyScholar also looks into site formation.