Cats love boxes due to their hunting instincts. In the wild, a box is an equivalent to a small hole in the ground or tree trunk. When the cat hides to observe or stalk its prey, these spaces will not allow other predators to sneak up upon it as the only way to spot or confront a cat would be to be in its direct line of sight. Cats aren’t fond of surprises.
Instead of being peeved at the idea that your cat may choose to spend time in a box or paper bag, why not use it to your advantage? You can use it to complement your cat condo or even make use of the boxes you have to make a cat maze of sorts. This would be a fun way for your cat to get exercise and the mental stimulation it requires.