We’re talking about physiology, not mechanical design. Nobody is going to write an ISO standard for the definition of FTP or training zones.
The best you’ll get are peer reviewed articles describing a specific approach, but we already have those in some cases (Critical Power Model for example). But most lay people aren’t going to wade through and comprehend all that.
Second best is someone like Coggan laying it out in plain speak for us (see the previous two links I gave). So we already have that as well.
So we do have standards. What other form of standard are you wanting?
If you’re asking why are there multiple competing models, there’s several reasons. First of all, even in science there’s always competing theories. The majority of scientists might agree that a particular theory is correct given what we know today, but that’s why science exists, to learn what we’re doing already know, which might prove those theories incomplete (Newtonian Physics vs Relativity).
And when it comes to lay person definitions, well that’s partially marketing at work (Sufferfest 4D fitness is just Coggan’s power profile chart) and partially human nature. I refer you to this famous XKCD comic.
My suggestion: use Coggan’s zones and Kolie Moore’s FTP test protocol. Two complementary standards used in one sentence. See, we do have standards.