https://www.youtube.com/watch?v=k7mmqwgBeLw
This is why.
Do you want a book built specifically to beat a meditating Forcemaster who turtles in her zone with 2+ Thoughtspores and 2+ Minds Eyes? If so... you'll probably want to change out 80% of your spell book.
In the video, it looks like Gravikor was too-little-too-late; the Necro needed to close distance earlier (preferably with some armor, regrowth, or other protections), then summon bigger zombies closer to the Spores, then take out the Spores ASAP. Use (Lesser) Teleports to get your zombies on top of the Spores and don't let up until the Forcemaster runs out of them. Then hope you can beat the Forcemaster in a damage race.
But that's a pretty unusual Forcemaster strategy (or at least it was before that video was posted). I'm not keeping up with the OCTGN tournaments, so if you're scheduled to play against Sailor Vulcan's Forcemaster and you're looking for advice, then you probably shouldn't be asking for book-building assistance in a public forum... or should be a little more transparent about it up front. (I honestly don't know if that's what you're doing)
All I know is a Necro zombie swarm usually wants to overwhelm an opponent with a horde as the primary goal, and as always, you will have to adapt to whatever your opponent might throw at you. It's hard to build for every possibly match-up, but you try to have a main strategy and a plan for some of your tougher/liklier match-ups and do the best you can. Minds Eye will be tough for a lot of mages. My best guess is to try to keep the pressure on. Dissolve the Meditation Amulet ASAP maybe, so the Forcemaster can't afford to power both Thoughspores (?).