Another “problem” with bear canisters, is that bears have really great smell. It may not ben that they can smell food through an air-tight container, but the container itself likely has trace amounts of food smell on the outside that got there as you handle it. As u/Volcan_R said, the idea is to make the food too hard to get, so the bear decides to put its effort elsewhere. So wether it’s hung from a tree, in a bear box, or behind an electric fence, (Yes such a product exists) even if the bear can smell it, it will decide it can’t get to it and learn to just move on.