They’re not dumb, they’re complacent. Just as many people are when things are stable for so long. They became too content. Much like real life. People grow complacent all the time and before they know it society and their lives change in the blink of an eye. We can be blinded by the truth until we lose the very things we could’ve prevented. Unfortunately it’s usually much too late. The same thing applies to the Jedi. They grew content, rigided, uncompromising, and egotistical. They became the very things they weren’t. Qui-Gon was the closest to what a true Jedi stood for in the last days of the Republic.
You can see it in these scenes with the Jedi:
And for reference to Qui-Gon: