Because WoW is usually most peoples' first online gaming experience. They don't understand that that is just generally how people are in online games, and on the internet in general, in an anonymous and not face to face setting. No, it was never better in the "old days", you just dealt with random people a lot less, so people had reputations and such. But even then, the crappy people still existed in droves. That's just kind of how online gaming is.
It's not really an excuse for crappy behavior, but when you've playing online games long enough you just sort of come to accept it and learn to ignore it and not let it get to you. From what I've noticed, there seems to be a lot of people who just haven't been into online gaming for long enough to understand this. Sure report them if they have done something terrible or whathaveyou, but if some guy is crap talking about your mom in Trade chat, you don't need to flip a table over it. I've seen people do and say crappy things all the time, in many games, not just WoW, but I've never got seriously angry over it. They are a random person on the internet, why would I?
I'm getting pretty sick of hearing all the time about how WoW's community will kill the game or something, or how it is declining because some guy ninja'd a level 50 blue item from you in a low level dungeon. Holy crap just get over it already.
Edit: And yes, obviously putting together groups yourself was better before they implemented LFR/LFD, but it still is! Seriously! I don't understand this complaint at all. "Yes before when you had to put together groups yourself it was better and not when I was somehow forced to randomly put them together when I could just as easily now put them together just like the old days". This makes zero sense.