if your mental model is that somebody writes codec specs and then fabrice bellard comes in and turns the specs into C, you are dead wrong. first of all, codecs are usually reverse-engineered, there is no spec. second of all, even when a well specified document describes the codec, that spec does not describe how to efficiently encode or decode with that codec. people like fabrice bellard develop the algorithms that do that.
It may also be worth pointing out, in terms of apportioning credit fairly, that ffmpeg has not been Bellard's project since 2004. The thing we see today is no more his project than GCC or Emacs are Stallman's projects.
Isn't that merely an observation of how lopsided media consumption vs production is on average?
In a normal standard development process experimental codecs come first, then those that have proved to work well, including having good enough performance, are described in the spec; after standardization there's very little room to "develop the algorithms" because nonconformant implementations would be useless.
Reverse engineering is limited to the abnormal case of having access to some codec but not to the standard that describes it.
there is A LOT OF ROOM to develop the algorithms. it seems that you are confused about what an algorithm is, since you seemingly think that there can be only 1 algorithm that can decode a given media file.
An algorithm is a sequence of steps and logic. You can create many different sequences to still get the same result.