celestials sort of levitate even when appearently standing. virtually all extremely large beings in marvel use anti-gravitons to stand to support their massive bulks.
the pillars themselves are compossed of ordinary stone built by the aztecs with eternal instruction/help to be the designated landing zone for the forth host. it was planned and constructed many many years before the celestials arrived and for that purpose alone.
I'm assuming ajak the member of the eternals that watches over the landing site had something to do with the construction.