Because not having players8 actually discourages me from playing with my friends.
Typically i have 1 core friend that i play with on a regular basis, and sometimes we can get 2 or 3 others to join in some nights. With only 2 people, there is literally 0 point in playing in a game together because all it does is make the game harder (drops increase only on players 3/5/7 but difficulty increases on players 2)
Even if i had a group of 8 friends to play with, it is actually more efficient for us to play 8 different games on players 1 instead of all 8 of us on in a single game and share drops with each other.
Not having /players x online actually discourages me from playing with a group outside of doing a baal run or something to get xp, or to occasionally rush a character for my friend(s) or vice versa.
Now, if i had /players x online? I could play /players8 with that 1 friend and it be more efficient to play together than for each of us to play solo because we could kill stuff much faster, and we will get better drops together.
It also prevents people from joining random games to snipe bosses. And playing with randoms is not a good way to get drops. You can be your a** that as soon as that ber rune drops suddenly nobody is willing to share.