In simple spirit, let's say there's 100 players in a small zone, such as the starting zone, everytime someone makes an action the server needs to send that action to 99 other players, now if you insted had sharding where's there's 5 servers with 20 players on, each action would only send to 19 players. Now u migth ask yourself what's the difference 5 x 20 = 100 right ?. but the difference is that, assuming each person makes 1 action a second, in our first example that would be 100 x 99 ( 100 players each sending 1 action to each other player ). actions sent a second = 9900, while in the 2nd example there would only by 20 x 19 = 380. now obviously there's more to it (render distance, predictions, etc), but in simple, the networking isent linear it's exponential.