Imagine all your information. What your character looks like, what they are doing, where they are looking, what they are wearing, everything that makes your character, your character. Take all that info and ship it to the other pc, but simultaneously get returned data on positioning, field of view, point of view, interactions, and do all that for 7 other players.
In the more complex graphical, more complex choice (not the same 3 terrorist models in csgo), more item type games, the amount of data passed back and forth can be staggering. And then start a fire-fight and all the bullets have to be accounted for, visible to all 8 players simultaneously at differing vectors, angles of view etc.
All that extra info, plus whatever you are doing with the mouse, cursor, keyboard etc means you'll need more threads/cores to split up the workload so that lag doesn't create a monster. If you want things to happen simultaneously, like your buddy shooting the guy about to stab you in the back, and he dies, then that's going to need to be processed over multiple threads. Waiting on single threads means dude actually got to stab you in your screen, but died first on your buddy's, due to event timer differences, lag.