Under the old lore it made sense why the Titans weren't around anymore. They showed up, remade the planet as they saw fit, and moved on to do the same thing over and over again all over the universe.
But now that we know they were searching for the "Prime Worldsoul", and we also know that they were fully aware they had found it with Azeroth, why would they leave? Especially considering it had at least four Old Gods locked up inside it — they might have never thought those would break free, but why risk it? What else were they up to while the most important thing in the universe just sort of sits on the back burner, not just out of sight but also seemingly out of mind?
But now that we know they were searching for the "Prime Worldsoul", and we also know that they were fully aware they had found it with Azeroth, why would they leave? Especially considering it had at least four Old Gods locked up inside it — they might have never thought those would break free, but why risk it? What else were they up to while the most important thing in the universe just sort of sits on the back burner, not just out of sight but also seemingly out of mind?