The thing with information is that once it is shared, it can't be unshared. Sure, blocking cross-site tracking would ostensibly make monopolistic accumulation of new data more difficult, but except for the most decay prone information, there is already a comprehensive profile established for a good chunk of the users, which can be milked for a good while. This is not even taking into account of backchannel acquisition of the missing data (i.e. through brokers) with the sweet sweet profits already made, potency of which is enhanced when joined with existing data (and therefore still creating monopolistic dynamics).