Perhaps they did - but in doing so simply created alternate realities where the Borg appeared in the past and assimilated Earth, leaving this one unaffected.
Yeah because that's how time travel has always worked in Star Trek. Jeez, you're a pain.
The Millenium trilogy was released in 1999
. It was written by Judith and Garfield Reeves-Stevens, who went on to write several episodes of Enterprise's fourth season.
Where do you think JJ Abrams and friends got the idea?