More specifically, Manwe. If Manwe and Varda and the rest were to just solve all the problems for Middle-earth, you'd have no plot. Furthermore, if you were in their shoes, would you be all that interested in fixing all their problems? I know I don't even like doing that for my daughters. Also, they'd "laid down their guardianship of Arda" with the fall of Numenor. Strictly speaking, it wasn't their job to fix all problems anymore.
Still, they did care about Middle-earth. So they sent five Istari - weaker spirits who were clothed in flesh and made to feel mortal cares and wants. They were intended to be messengers and encouragers of the good nature of the Free Peoples. They were forbidden to challenge Sauron's power directly. In extremis, one of their Istari could call on the Eagles of Manwe for assistance, as was done a few times during the novels. But any of the Istari calling on them to solve the problem by flying over Orodruin and dropping the Ring into it - I don't think they would have responded to that.