It was assumed (at least by me) that when Qui-gon Jin told the council he found the person mentioned in the ‘one who would bring balance to the force’ prophecy the Council considered having a new champion for their cause, but were only worried that if it was indeed him he just wasn’t ready.
I now think otherwise.
During the Old Republic the Jedi Order was primarily responsible for bringing order, justice and peace to the galaxy and except for a few independents on the Outer Rim and issues with a rogue Trade Federation in league with a bunch of anthropomorphic cockroaches they were by and large incredibly successful. The Senate brought democracy and enlightened rule to practically the entire universe. To bring balance would be in fact to add disorder, injustice and war. Mace and Yoda surly understood and sympathized with the ebb and flow of nature and to borrow a D&D term would be considered to have a True Neutral alignment. So they allowed this child to grow up, trained him, gave bullshit ‘hard to see the future is’ answers when confronted with the dire warnings of the Republics impending doom and all this with the full knowledge that this kid would contribute to the slaughter and subjugation of most of the universe including the Jedi Order itself.