It's not - there are two entities (FSF and OSI), they happen to have similar definitions, and a number of important organizations (Linux distros, vendors with special plans for open-source users, employers with open-source contribution policies, etc.) all happen to have specified their definition in terms of one or both of those because they're pretty good definitions. For instance, Ubuntu's licensing policy has text that pretty closely matches the OSD's, but does not defer to the OSI for the definition. (And Debian's, of course, is the text on which the OSD was based - any changes to the OSI-maintained OSD would not necessarily flow back to the DFSG. Debian also considers some of the FSF's own licenses non-free, for good reason IMO.)
Concretely, if the OSI were to change the OSD in either a way that included things like the SSPL or that included things like the Hippocratic License, I think there's a good chance that my employer's internal OSS policies would change to say "the version of the OSD before 2020," because it's not clearly in their interest to contribute company-owned code to projects under those licenses. On similar grounds, I'd expect companies that provide free services (hosting, CI, etc.) to F/OSS projects to say "these licenses don't count," Linux distros to not universally agree on including them, etc.
So you're already in a place where there's no sole authority: you need to start by convincing everyone other than the OSI that a new sort of license is actually a good idea and the change you want to make to the OSD is actually something that they, too, should consider "open source." And once you get to the point where enough people agree with you, the OSI isn't going to be in your way in any practical sense.
The GNU Free Documentation License (GFDL). It's generally well-intentioned, but it places some weird restrictions on modification and reuse. In particular, it allows the author of a text to define "invariant" sections of a text which cannot be removed or altered in copies of the work, and automatically applies this restriction to sections of a document with certain names (like "Dedication").
The GFDL also theoretically forbids users from storing GFDL-licensed documents on encrypted storage, as the license states that "You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute". I don't think that reading was intended, but the license doesn't clarify further. :)
I would say, by and large, every conversation about a new license is near immediate dismissal because the OSI hasn't approved it. The conversation that should happen rarely does because the OSI has decided they have no desire or need to make open source development sustainable.
Concretely, if the OSI were to change the OSD in either a way that included things like the SSPL or that included things like the Hippocratic License, I think there's a good chance that my employer's internal OSS policies would change to say "the version of the OSD before 2020," because it's not clearly in their interest to contribute company-owned code to projects under those licenses. On similar grounds, I'd expect companies that provide free services (hosting, CI, etc.) to F/OSS projects to say "these licenses don't count," Linux distros to not universally agree on including them, etc.
So you're already in a place where there's no sole authority: you need to start by convincing everyone other than the OSI that a new sort of license is actually a good idea and the change you want to make to the OSD is actually something that they, too, should consider "open source." And once you get to the point where enough people agree with you, the OSI isn't going to be in your way in any practical sense.