Tolkien answered thatanswered that himself:
"He is in great fear, not knowing what mighty one may suddenly appear, wielding wielding the Ring, >andand assailing him with war, seeking to cast him down down and take his place. That we should wish >toto cast him down and have no no one in his place is not a thought that occurs to his mind. That >we shouldwe should try to destroy the Ring itself has not yet entered into his darkest darkest dream."
Why didn't he destroy Orodruin? Morgoth couldn't do it and he was the mightiest of the Valar. Why should Sauron, a Maia, be able to? I also imagine it may have something to do with its creation by Eru Ilúvatar. Notice that there are very few geographical things are actually destroyed in the mythoi. Even the lands that lie under water still exist, but are difficult to access. :D
