Because as soon as we form a “collection” (ie. association / organization / union), any power we have begins to erode due to squabbling about “who should be in charge”, and “who gets assigned to which games”, and “who is “better” than who” (and should get paid / assigned more).
Leadership takes (a cut, a rake, a skim, a fee), and invariably and inevitably, someone gets butt-hurt about the operations of said collective. That someone forms their own entity, and subsequently under-&-out-bids the existing leadership in due time, likely sniping umpires from the previous entity by enticing with “better” pay and assignments.
Then you get the “loyalty wars”.
The machine churns on.