They just have a really unintuitive economy where they basically only need to work once or twice to make up for the cost of all the times they don't contribute any benefit.
For extra fun, you sometimes can make ideal filters with no false positives, if you know your possible elements ahead of time and you don't insert too many of them. (E.g., for 20 elements, you can construct a 12-bit code where there are guaranteed no false positives as long as you insert at most two elements.)
