Because a lot of people play online multiplayer.

I hate 2 say this,but I actually agree with you on this. I guess there's a first time for everything. 

I think multiplayer can be really fun. I never even played any kind of multiplayer,until this gen. I love playing the battlefield multiplayer. Sure there's the cheaters and people that like to insult everybody. I always mute everybody most of the time. You just have to take the good with the bad. Multiplayer is as fun as you want it 2 be.

I Never mic with people online and I still pretty play by myself when I play multiplayer. Sure I'm on a team,but I'm still doing my own thing. lol

Because it's fun to hear 10 year olds insult you with things like, "You are gay!" or ,"NoobTuber!"

Who doesn't like that?

I don't do much MP online, but my son(15 YO) plays the heck out of it with his friends. Hard to get them together, so online works best for them.

Online multiplayer is great because you're playing with actually other human beings and not computerized AI. I love anything Online.
The online feature in Journey is what made Journey such a great game, because you're playing with another human being. Single player games get boring FAST. You beat it, maybe twice or three times, but then it sits and collects dust.
Multiplayer adds re playability to games so you don't fully waste your money on a game that you'll only play twice.

I think it's funny,but if you don't want to hear it,it's called using the mute button. Could'nt get any more simple then that.

Playing multiplayer with friends even if they live in another state is a pretty big deal for me. The main reason I play my 360 more than my PS3 is because I know way more people who own that system than PS3. I don't like playing COD or Battlefield by myself, I like to join up with my friends and play with them. Playing with strangers is more of a risk but you can meet some pretty cool people. If online multiplayer isn't for you, then none of us are going to change your mind. But it is a pretty important factor for a lot of us.

If I'm playing a co-op game then I want the person to be there beside me or at least be a friend of mine. I don't enjoy playing with random people that I have no connection with. Competitive multiplayer I'm fine with. Sometimes I'd like to play against someone whether it be a fps, fighting game, or whatever. Usually if I'm playing with people beside me, it's only 1 person, and I don't enjoy 1 vs 1 that much (as I usually play someone who's much better than me and I get slaughtered haha).

tl/dr
Co-op: I want them sitting next to me.
Multiplayer: I like online for greater number of people.