Speaking personally, one of the reasons I don't play in Open is that there simply isn't a good enough reason to do so. I can't say that I would be particularly bothered by the phantom menace of PVP griefers; with so few human players present in such a large volume of space the threat is more imaginary than real. However, at present there is no reason to participate in Open play other than to potentially fight human players rather than lines of code dressed up in pixels. The originator of this thread paints a picture where human players add depth to the galaxy and enrich each other's enjoyment, but frankly I am unable to see how. Waving from your desktop when passing another CMDR like a bus driver greeting another is not substantially improving anyone's experience, unless you are unbelievably lonely. There is simply not enough to do with other human players in a coop sense and little reason to try. I have seen videos of players teaming up but it seems forced and rather pointless. In a combat zone, it is rare (so I have seen) to be engaged by more than one enemy at a time, robbing you of the necessity to have a wingman stationed by your side to watch your back. With no facility to share the proceeds and benefits of kills, why would another player want to avoid trying to take the final shot themselves or hunt their own target? Equally there is no point in players teaming up to act as an escort for a freighter. On the rare occasion that the escorted player is successfully interdicted (most likely not by a human player) and drops out of SC, the escort would have to very quickly drop out too and hope not to be thousands of KMs distant faced with the difficulty of finding their buddy in the vastness of normal space. As for conversing with other CMDRs: I can see why many don't bother to respond to requests. What is there to discuss beyond the topics of "Seen many players lately?", "Been anywhere interesting?", "Is that a Cobra MKIII? I have one of those (along with about everyone else)" and "Any idea where I can buy/sell X?".
TBH part of the problem is that ED is simply too big and therefore too empty. The encounters that might in other games take place in normal space take place mostly in SC, at such speeds that by the time you even notice another player, he or she has already disappeared off the radar and either dropped into the empty nothingness of normal space or jumped system. The problem (as other people have stated) is that ED is essentially an SP game trying to be an MMO and currently it just doesn't work. Half of the players are at best ambivalent about the MP elements and there just aren't enough things to do to encourage them (and as stated I include myself in this group) to alter their perception of the game and their intentions. Perhaps all the MP fans are really just feeling a trifle lonely: "In space no-one can hear you scream", in ED, there's no-one there to hear it.