Plane and simple what seems simple and no big deal to a novice might actually be technically impossible for the publisher. Even doing it by hand could be impossible depending on how the process is automated.

For example, you know those cards that fall out magazines usually advertising subscriptions? Those are called blow ins if I still remember my terminology and there is a machine that actually uses air to blow those in. Even if the magazine is being printed on a press that allows this, you still might have limitations on the size and types of paper that can be blown in.

In real life things are much more complicated then the average person assumes.