I don't think that they are all dumb. There are few that have had IQ's exceeding double digits. Usually, they become pretty full of themselves believing the press clippings about their genius. LaRussa is the prime example. A smart guy that lost sight of the fact that it is a pretty simple game. He would start mixing and matching relievers in the 6th inning to the point where he slowed the game down to a crawl. Cardinal games became unwatchable for me with his double switches and the overkill of the mixing and matching. As a whole, my opinion is that managers have never been a very smart bunch, but as you pointed out, it is not a job requirement. I don't know why it seems to bother you that I have a low opinion of the intelligence of baseball managers and that I refer to them as the gym teachers of baseball. I guess that it might surprise you that Hall of Fame manager, Casey Stengel (aka "the Old Professor") did not enjoy the respect of a lot of his Yankee players, many who referred to him as an idiot. He was the butt of many jokes by the players. For me, it is part of the fun of the game to make fun of poor managerial decisions.
I hope that it doesn't offend you that I also believe that generally politicians are disingenuous and dishonest.