In most MMOs I play I am not in a guild (I currently rotate between 5 MMOs as my "secondary to DDO" game). First, every single guild I've ever been in has been full of drama and most of them have died from that drama. I'm still in a guild in DDO but that also almost died from drama. The only reason I'm still in it is because a handful of us were still standing and all the drama-causers left the guild in a huff.
Second, because I like to rotate between games I don't feel its fair to join a guild when I'll only be playing for a few months and then switching to another game.
Sure, its possible to join a guild that doesn't have drama but that should be a choice. And no, I don't get more from a single player game than all those other MMOs I play without a guild. I still chat with people, trade with people, do events with people and occasionally run content with people. All without being in a guild.