Because they're scared of us. And no, I'm not kidding.
In Voyager: Dark Frontier, the Borg Queen has a tête–à–tête with her new gal pal, Seven of Nine. She describes how the Borg's prior attempts to assimilate humanity via direct assaults have proven to be abject failures.
Since each attempt has resulted in the transfer of technology to humanity (making each subsequent attempt that much harder), the Queen's next step is to try something rather more subtle.
QUEEN: Your knowledge for the target species is invaluable. Species five six one eight. Human. Warp capable. Origin, grid three
two five. Physiology inefficient, below average cranial capacity,
minimal redundant systems, limited regenerative abilities. Our
previous attempts to assimilate them were all direct assaults. They
failed, so we've created a more surreptitious strategy.
SEVEN: You intend to detonate a biogenic charge in Earth's atmosphere.
QUEEN: It would infect all lifeforms with nanoprobe viruses. Assimilation would be gradual. By the time they realised what was
happening, half their population would be drones.
SEVEN: Inefficient. The virus would take years to proliferate.
QUEEN: We've waited this long
Based on the results of the prior attempts (Cubes being repeatedly infiltrated, the Borg Resistance being created, Borg ships being taken over by an android, etc) the fear is presumably that a strategy that involves sending a fleet of Borg Cubes to attack Earth could easily end up with a fleet of freshly-liberated Borg Cubes turning around and attacking the Borg.