Yoda and the rest of the council don’t join them because, yet again, they’re blinded by their arrogance. They don’t believe the Sith could have returned without any of them knowing about it first.
Not true.
“We will use all of our resources to uncover this mystery.”
“This could be the clue that we need to unravel the mystery of the Sith.”
The real reason is that they cannot do so without the Senate’s permission.
“I can only protect you. I can’t fight a war for you.”