Because of Drones, that's why. It's why Gallente is the scum of the universe.
For those of you that need more than that brief explanation. When a system or node is overloaded in EVE, time dilation (TIDI) kicks in. That basically slows game-time in real-time, so a minute in-game is actually 10 minutes when the game is running normally. The objective here being to deal with massive volumes of data, give everything a chance to process so you don't have a fleet of 5000 people waiting and killing anything that jumps in because their grid is pre-loaded, etc. Basic idea, anyway.
It's a massive complaint from the serial whiners in EVE that TIDI kicks in as they conveniently forget that in the early days you'd have a server crash rather than just a slideshow of very slowly moving spaceships. Which is roughly appropriate for spreadsheets in space.
Anyway, this picture illustrates why. Because even though there are what, 4000 to 10000 pilots battling it out, each of them controls 1 or more drones. And the drones, as you can see them swarming over the screen, are like little pilots also with their own actions, ideas and movements and things the server needs to calculate.
Frankly, I'm astonished the servers don't melt. I can't think of another MMO that deals with this volume of processing in a single, open event on a single server. (Be that server a "virtual" kind of server)