The Jedi hadn't faced a true Dark Lord of the Sith in a thousand years (Maul being an apprentice and Dooku more of a fallen Jedi). On top of this, they were going against the most powerful Sith in history. Ep III novel states that the Sith had evolved over the 1000 year gap while the Jedi maintained complacency in terms of their philosophies and teachings. The novel even states that Yoda acknowledged that he couldn't defeat what the Sith had become. Yes maybe Mace and co were a little arrogant but I think saying they were ignorant more describes the situation.
I totally agree, and ignorance and arrogance often go hand in hand. Just look at their reaction when Qui-gon first suggests he faced a Sith (Maul), and Mace replies with , "I don't think the Sith could have returned without us knowing."