Like people have said - the tools are there but people don't co-operate. But it's just a vicious chain reaction, i used to always say "hey" whenever i joined a LFD group but nowadays i just don't waste my time typing, if someone else does then i will say hi back but if not then i'll just focus on killing stuff as quickly as possible. There is on the rare occasion when i do find an entire group that does chat to one another and imo it makes the dungeon experience so much more fun but it's not something to expect going into an instance.
The majority of PVP communication really is just "Goto X" "Everyone sucks but me" "Y is a noob" etc etc. I just don't bother any more and im gunna guess others feel the same and because so many don't bother talking it just makes others not bother and so on.
As for open world content, the only time i ever have a proper social interaction with someone is if i end up seeing them on more than 1 occasion while leveling - we tend to just group up and do some quests together but a lot of the time i meet people who group up for 1 quest and then they're off without saying a word. The social aspect mainly lives within Battle.net and particular guilds. But it's all down to the individual, personally I like social interactions more than playing in silence but I can't just rely on friends to be there every time i want to do something so sometimes just gotta suck it up and go with it.