You are by far not the first to have that thought. The problem with developing for multiple threads is to keep things in sync. This is especially important here, where you have several thousand players on the same grid that all want to see the same things happen on their screens. Having everything run out of sync is much worse than having everything run slow AF.
And then there’s the fact that the game logic from all I know is written in python, and developing multi-threaded applications in python really is no fun at all.
I can imagine that it’s possible to give each grid its own thread, but this would for one not help much, as most large battles all happen in one grid anyway, and it would likely cause problems when mutiple grids start merging.
What definitely can be (and is being) done is to keep as many things as possible that aren’t immediately relevant to flying in space out of that one main thread, and to reduce the calculations this one thread has to do, for example by merging swarms of fighters into a squad, that to the server now is just one entity. But this has limits of course: Many things simply need to be in sync at all times (i.e. ship stats, skill bonuses, hitpoints, damage dealt, target locks, …).