The Borg stopped making sense as an enemy when we saw just how many cubes they actually had, not to mention their ability to time travel whenever they wanted like in First Contact.
As an aside, I suspect the 'real Borg' (i.e. their first incarnations as unitemporal hive minds without the Queen) was simply too difficult to dramatise.
In real life, they'd be horribly threatening. In film, writers/studios want a nice, neat villain character, who is nasty enough to be a good antagonist but not so nasty that they do what's necessary to win (e.g. go back in time to right now
and assimilate the bejesus out of Earth).
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.
The Deep Space Nine "Millenium" novel trilogy mentioned the Klingons trying something similar in the 23rd century - war fleets were built and slingshotted into the past, never to be seen nor heard from again. Some suspected something went wrong and they were destroyed en route, others believed they conquered Earth in alternate realities.