All of the points you mentioned in answering this question are valid. But you omitted something that is equally valid. The question “How come archaeological ruins are always underground?”, and your answers to that question, imply a fact that’s not necessarily true. The implication, put concisely, is that “old stuff, especially old stuff left by human cultures, is always buried.” You then proceed to give reasons why this would be so. But you don’t question the implication. Here’s what I think. Old stuff is left unburied all the time. Maybe less than buried stuff, maybe more. But items left unburied are, by that very exposure, subject to all sorts of elements and factors that prevent preservation through the ages. Rain, wind, sandstorms, human and animal scavenging, centuries of being stepped on by animals – all contribute to the disappearance of this material, leaving the buried (and thereby protected and preserved) stuff for archaeologists to find and study.
Maybe this seemed self-evident, so you didn’t bring it up. But I think it deserves mention.