Because that is the definition of "Open Source"[0]. As was already said, "source available" is the correct term here.
[0] https://en.wikipedia.org/wiki/The_Open_Source_Definition
They are a single organization, that have done a tremendous job at trying to come up with a global and shared legal framework to which people can license code under. They have gone so far to come up with a pretty good definition of "open source", but not the definition.
This would be equivalent to saying that "Freedom" is defined by the US Constitution or the Canadian Charter of Rights & Freedoms. It is not, those are both examples of a legal definition of freedom, but neither are the sole authority for the global and cultural concept of "Freedom"
The idea of freedom existed before both documents. The idea of Open Source was proposed in 1998 [0], and the OSI was created to define it in the same year [1]. This is not at all equivalent.
[0] https://en.wikipedia.org/wiki/History_of_free_and_open-sourc...