You're forgetting that Tom Riddle actually did study at the monastery before he destroyed it to deny that training to his enemies.

Voldemort is especially violent and comes off as stupid, but he's just one of Tom Riddle's characters, and if you consider their actions as a whole they're smarter than they appear, on purpose.

There is a classic trick that card counting teams use to avoid detection. If one person shows up, and bets conservatively until the cards are in their favor, and then immediately starts making huge bets, then it is obvious that they are a card counter and the casino can throw them out. But, if that one person betting conservatively simply leaves the table once he thinks the deck is in his favor, and then someone else comes in wearing a loud Hawaiian shirt and acting like a "wild and crazy" risk-lover, then it just looks like someone risk-averse has been replaced with someone risk loving, and neither looks like they're counting cards.