Because your criteria for labeling it "objective" is based on common agreement, not whether or not it's actually objectively true.
Maybe the problem here is grammatical. What exactly do you mean by "objectively agreed upon"? That phrase can be interpreted in a variety of different ways, and each interpretation would give it a completely different meaning.
For example, one person might assume that it means that it's objectively true that people agree with it, while a second person might assume it means that people agree that it's objectively true, while a third person might assume that it means that people who consider the question objectively agree that it's true.
So what exactly do you mean by "objectively agreed upon"?
It has to do with the actual content of the context.
The question asked was "is subjective knowledge possible in any sense?" which in the course of my answer another question was asked:
"What is the difference between subjectively, inter-subjectively and objectively agreed on? How do you explain your usage of objective instead of another word or no word at all; i.e. "that bit is agreed upon"?"
So to me it is better to use the word 'objective' in relation to the word 'subjective' rather than the words 'generally' or 'universally', especially when working with the fact that we each are experiencing the physical universe subjectively.
To my knowledge it is not argued that we are living on a planet in a galaxy in a universe. The 'we' are all subjective positions which agree to the objective position (which as you say is generally or universally' agreed on.
That bit is objectively agreed upon as far as I can tell. Otherwise the question might have to be:
"is objective knowledge possible in any sense?"
So the problem here might not be grammatical, but the language itself.
Or, as you say, to reduce the possibility of subjective individuals assuming a number of things from the word being used, that a more appropriate word such as generally or universally need be used.
So then...would those words be helpful in trying to answer the original question?
"is subjective knowledge possible in any sense?"
We know objective knowledge is. Certainly in relation to the earth, solar system, galaxy and universe and subjective experience to do with those things...objective knowledge is possible.