Then again, the Jedi for some reason never figured it out on their own either, despite being in the same room with him mutiple times.
The old EU's reason for that was that the generations of Sith had steadily "polluted" the Force in the galaxy with minor conflicts and rituals. This had the result of slowly clouding the Jedi's sensitivity towards the dark side. Have you ever had an experience wherein a background noise you were filtering out stopped, and suddenly you realize how much more quiet it is? Like an airconditioner? It's kind of like that. The Sith slowly stirred up this additional noise in the Force that the Jedi just automatically filter out, and thereby created a blind spot to hide in.
I don't know how much of that explanation survived Disney, but it is actually alluded to twice in Episode 2. First time: the meeting between the Jedi Council and Palpatine after the assassination attempt on Amidala.
Palpatine: Master Yoda, do you really think it will come to war?
Yoda: Hmmm. The dark side clouds everything. Impossible to see, the future is.
Think about it. Palpatine is in the final phases of
launching this war that he'll use to dismantle democracy and slay the Jedi. He is
taunting Yoda
to his face in this scene. And getting away with it.
Later, in the Jedi Council chambers after Obi-Wan reports the existence of the clone army.
Yoda: Blind we are, if creation of this clone army we could not see.
Mace Windu: I think it is time we inform the senate that our ability to use the force has diminished.
Yoda: Only the Dark Lord of the Sith knows of our weakness. If informed the senate is, multiply our adversaries will.
States right out that the Council knows that their cognition is impaired, they don't know what to do about it, that they know the Sith Lord must know, and they're trying to keep a lid on it to maintain their myth of infallibility.
The prequels don't get as much credit as they deserve. They're really composed of two plots: the first is a classic Greek tragedy (in a Shakespearean mold) of the fall of a hero through hubris. The second is a political thriller of post 9/11 themes (which is actually pretty damn impressive when you consider it started before 9/11) wherein an evil mastermind uses proxy wars and terrorism to destroy liberties and achieve totalitarian government. The first plot line was bungled so badly that nobody gives the second plot line the credit its due: it was actually fairly well done (not masterfully, but fairly well), and is the most interesting big concept that the movie series ever tried to handle. Unfortunately, I think fan pushback on how it was presented in the first movie led to it being under represented in subsequent movies, which put even more focus on the botched heroic tragedy. If I give the Clone Wars show credit for 1 thing, is that it brings back emphasis on that plot line.