Probably because Vger and the 'soul'(for lack of a better term) of the Borg share(d) the same problem:
They were entities that understood their own existence in the universe,but that's ALL they understood.
In V'Gers case,all it understood was the greater universe:and that it had seen all of the universe it could ,so unless this 'creater' had more instructions,it would become purposeless,a terrifying prospect for a sentient machine.Its 'evolution' with Decker/Ilia allowed Vger to move past its machine limitations to embrace making its own reason to exist.
In the case of the Borg,it was motivated by an urge to assimilate and conquer all,an urge motivated by a corrupted intelligence that had long since lost grip of everything but the basic calculus of survival, to seize and possess all that it can by force,or die.
That's why the Borg do not negotiate or bargain with assimilation any more than a soldier facing death on a battlefield will toss his assault rifle.As far as the collective is concerned unless it assimilates it will perish and it would rather integrate biological distinctiveness to ensure its survival ,than be crushed by that distinctiveness.
This is also why they go scorched-Earth(excuse the pun) against the Federation,as rather than pursue negotiation the Borg percieved the threat of the UFP's resistance as a threat to the Borgs entire existence,even though logically Starfleet,Romulus,and all the Alpha Quadrant united could never wipe out ALL of the Borg .
There's just one problem with operating on survival mode:What happens when you've won the contest?
Once the soldier kills the enemy,he goes home to find another purpose in life.
Once the lion kills the cheetah,it goes back to its pride for another purpose.
Once the Borg have assimilated all,that's it.There is no other purpose or reason for the Borg to exist once its former enemies have been assimilated.
Just like Vger didn't have a reason to exist once it had explored the universe and returned to Earth.
The Borg,like Vger,needed to banish its limited (and severly corrupted) original motivation to discover a REAL collective in the form of the Caeliar,not the counterfeit one established millenia ago from fear and primal survival.
Sorry if that was too long for yall