The biggest companies in the video encoding industry, the ones with vast expertise in video patents, have a significant financial interest in making sure that no one else has a patent claim on their cash cow.
Oh really? That wikipedia page says that Microsoft won on the MP3 related patents (on appeal), in part specifically because they had licensed MP3 technology from Fraunhofer's MP3 pool. (Getting your $1.5B damages dropped because you spent $16M on licenses is a terrific bargain too). The parts where Microsoft has lost are on user interface patents unrelated to MP3 coding.
It's a lot like buying mob protection.