MPEG plays a clever game with their standards. A standard like MPEG-2 or MPEG-4 Part 10 (aka H.264) doesn't just refer to one standard but rather a whole series of standards published over an extended period. Patents are pooled by standard with deliberate ambiguity over which parts of the standard each patent actually covers.

This lets patent holders spread FUD over whether earlier parts of the standard are actually patent free even after 20 years have passed since the original publishing.

In the face of patent holders threatening a costly legal battle, companies choose to continue paying licensing fees even on standards which plainly should be out of patent protection.