there'd be nothing to get good at. Like the mimic in sen's fortress - it's a one off gotcha moment to waste time
Agreed. There are rules and there are exceptions.
A bonfire is the equivalent of a checkpoint, and it's a safe guarantee that the player's position and progress will be saved.
Because of that, people might come to the conclusion that it's the best way to surprise the player, since it's in the safest moments that people usually let their guard down.
A chest is just a container for an item. It's contents are revealed only when the player opens it, so whenever the player sees a chest in most games, he reads "reward", and his immediate reaction is to open it without thinking too much about it.
Adding a trap to the chest breaks this rule in his mind, adding the possibility of not only reward, but punishment as well.
Some people might say it makes the game more challenging, because experienced players will be more careful and will learn the hard way.
While true, not everything needs to be challenging. An experienced game designer will put a lot of thought into what should be challenging and what shouldn't be, because a simple change can make or break the game.
When a chest has the possibility of containing a trap, this simple fact adds stress and tension to the interaction. Now the player needs double the amount of effort to ensure that it's safe to interact with the chest.
With a bonfire, it's the opposite experience that's intended, as it's the only safe haven for the player. The designers wanted to enforce this rule that there's no tension and stress associated with interacting with a bonfire.
In other words, simple things should be kept simple. Adding more layers of complexity to an interaction doesn't always equate to a better game experience.