The best explanation I’ve heard was that it was memorial/warning of a cataclysm that wiped out a large portion of mankind. Graham Hancock has a pretty good explanation of i that it was likely warning people to watch out for comet/meteor strike during certain times of the year.
Yeah, because they wouldn't be building shelters for themselves, even though they're smart enough to allegedly have some kind of precocious astronomical knowledge. GH is a buffoon.