Video codecs, like any other software, ultimately do have to compete on merits other than cost. In this case, it might not be "hard" to make a video codec, but

Video codecs, like any other software, ultimately do have to compete on merits other than cost. In this case, it might not be "hard" to make a video codec, but it is hard to make a codec that is competitive with anything of the leading codecs from the MPEG LA. Bear in mind that your new codec needs to likely be the best or leading in most of several hundred metrics: power efficiency of encode/decode, compressed size in transmission, video/audio quality measures of many sorts, flexibility of storage in various encapsulating containers, etc. etc. It's a rather long list, and all the while you have to dodge the efforts of many many patent trolls waiting to sue you over infringing their patent. After having gone through that effort, the costs of it better ultimately justify the outcome, and the MPEG LA licensing rates are very carefully to get you to avoid just this outcome.