There is more than one type of mythology that ascribes a certain kind of vital essence to a rock. The Niobe rock of Greek mythology, the weapon that allowed David to overcome Goliath in Biblical lore, and Celtic mythology is no exception to this rule as can be evidenced from Merddyn delivering prophecies from a a chair on a rock in Welsh mythology. In the context of mythology, a rock is more than just a rock. It represents the native soil and being able to draw on its powers. The rock seems to release Excalibur for Arthur's sake because his potential to rule as the Chosen of the Earth is recognized by the powers that be, represented in part both by Vivian and Morgan. Arthur seems to stand out in more than one respect from the other warlords of his time. The only one able to defeat the Saxons, the only one worthy of Excalibur, one of the few said to return upon a n hour of need, even his conception through deceit is reminiscent of those themes in mythology where the hero is ascribed a divine father. Whichever way you look, Arthur himself is a type of a folk hero, and as such it makes senses that his legitimacy is established by the magical rock and thus the Earth itself.