The Borg are a Hive Mind, the Collective is made up of the COLLECTIVE minds and wills of the Borg.
If the Borg are entirely made up of people that were forcibly assimilated, than that means the Collective is made up entirely from the minds of people who didn't want to be Borg.
This clearly isn't the case, meaning the Collective must have more minds in it that WANT to be Borg that they can drown out the wills and minds of the ones who DON'T.