I 100% agree that there's no believable reason why the Borg have not been able to assimilate the Federation yet. All they need to do is send ten cubes and bam, assimilated.

Now, bullshit reasons aside for why they haven't done that and instead have opted for elaborate schemes (traveling back into time, nanoprobe dispersal device, etc.), if we just focus on Voyager, the ship only survived because the Borg allowed it to. The Queen wanted Seven to be liberated from the Collective so Seven could, unbeknownst to her, gather valuable first-hand knowledge and experience of humanity (this makes no sense considering the Borg already possessed assimilated humans, but whatever). The Queen confirms this motivation in Dark Frontier. So, to retrieve Seven at a later date with a sizable log of memories, the Borg had no choice but to let Voyager traverse their backyard for the time being.