The lore reason is essentially that defeating Sauron was mankind’s coming-of-age story (the age of elves was ending, and mankind was set to take over control of middle earth), and having a bunch of maiar come in and wreck Sauron wouldn’t teach men to stand up for what’s right. Instead, Eru told Manwë to send the istari to guide men and elves to defeat Sauron on their own
The “real” reason is that it wouldn’t be a very good story if Manwë just sniped Sauron from the hidden West with magic
To be fair, the istari were diminished Maiar who weren’t allowed to use their full power, and Sauron was a full Maia with no qualms about flexing his true strength.
Had Manwë been given the license to send just Eönwë, then Sauron would’ve been rekt in a year tops
As stated in unfinished tales, Gandalf didn’t know that Bilbo would find the ring on the adventure. He originally wanted to help Thorin since having dwarves in the lonely mountain would prevent Sauron from attacking Gondor and Lothlórien from the north. The ring finding it’s way to Gollum and then Bilbo was almost definitely due to slight meddling from Eru (just as Gollum’s death was due to Eru loosening the rocks under his feet) so Gandalf could orchestrate the fellowship’s journey.
And she did! The ents were explicitly made to protect the trees from the other races of middle earth