The collective has been in existence for over 900 years.
They don't expand like a virus.
There's thought and contemplation to their method.
The Borg invite cultures to sign up who are worthy.
They don't want everyone.
If they took everyone, it would be serious diminishing returns for the Borg drafting endless reams of losers and for the new Borg Pledges too who are all signing up to a crappier and still yet crappier definition of the collective for every Pakled wannabe tosser they feel compelled to induct.
The Borg assimilate cultures, not individuals.
The Borg would have ignored Voyager completely if Janeway hadn't been such an adversarial and aggressive prick.
Do you want to know why the Borg have not really overcome the universe?
Nothing out there is still truly that startlingly unique.
All these new worlds with new species with the same old technology.
The Borg are not redundant.
It's an inversely proportionate relationship.
For every species they assimilate there's so many more nearly identical races out there which they now don't have to bother with who get a free pass.