So there were five godlike beings sent to fight Sauron. Only one of them did his job.
I need to reword it.
You are the big cool powerful god. One of your servants, a minor much less powerful god does bad things to the world. So you send five your other servants just as powerful as the bad one to deal with him.
A lot of time passes. Three of those spend their time chilling. One joins the bad one. The last one turns out too weak. Who solves the problem? Four hobbits.
You really should reconsider your politics after that.
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
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
It's complicated but basically the gods learned that flexing their full strength caused cataclysmic events. Not ideal. But in doing so they got rid of Sauron's daddy, so it wasn't all gloom. But it did mean they weren't keen on the idea of going rampage mode to deal with a lesser threat.
Isn’t much of the power of the Maiar in diplomacy and setting events in motion? Gandalf was as much of an interloper and manipulator as he was anything else, and his hiring Bilbo as a thief was the penultimate piece of his mission, as inadvertent as I’m not entirely sure it was. Right? No, really, I’m kinda asking, I don’t know for sure.
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.
Wait till you learn about Melkor! He's a Vala, or one of the Valar, which is a higher order than the Maiar, and was basically super-Sauron from the before times
And he was scared of Ungoliant, and we don't know what she is, besides nasty, and hungry, and shaped like a huge spider (well, spiders are shaped like her, probably).
(He also got his foot almost cut off by an elf in single combat and walked with a limp ever after — well, at least until he got his hands and feet cut off by the rest of the Valar, I suppose —, but elves were mighty back then.)
Do we know that the Istari who go east were just chilling? I thought they were trying to rally men in the east to fight Sauron. They might even have fought some of his troops in the far off east during the Battle of the Black Gate? Or were those just fan theories and never actually confirmed?