why kick that beehive before it's ACTUALLY necessary?
Because by the time it's actually necessary, you're fucked. Case in point, if Sotomayor had resigned last year, her replacement would have sailed through, and there could be a 40 year old solidly liberal justice in her place, penning equally liberal opinions and poised to continue doing so for decades.
But she didn't, so if she acts now, her replacement would get caught up in "senate can't nominate in election years for reasons" BS. Big political fight, but one that's winnable since Dems ultimately hold the Senate.
If she puts it off yet further, she would have to continue for the next 4, possibly 8+ years. And maybe by that time the democrats don't have both the presidency and senate anymore, so her replacement is a less liberal consensus candidate.
Failing to think strategically is an extremely bad idea when it comes to institutions like the Supreme Court.