The Duty Finder is a means to an end for the most part, and that extends to the people you get grouped with. Its a quick resource to get things done, not to have a chat. Yeah, people can talk to each other and such but the majority just want to clear whatever it is they queued for and move on. Especially since beyond the first run or two of something, everyone should know what to expect so we go through the motions. Every. Time. Not even the revamped Diadem gives much reason to communicate. Chase FATEs, maybe mark the map for which FATE is next...

As already mentioned somewhere, the dungeon/battle model of FFXIV is not very in-fight chat friendly. Once you get started, people are starring at status/health bars, positionals, floor AoEs, cooldowns, etc. No real time to chat if you aren't lazy auto-piloting. And by the time everything dies, there's no reason to chat in-between groups of mobs unless someone does something somebody doesn't like.

To add to those, the chances of you ever seeing any of these people again are slim to none, so no point making an impression or getting attached to anyone since they're probably not on your server (unless you are actively looking to leave yours). Sure, "hello" and "thanks/goodbye" are etiquette, but I've been in way more DF runs where nobody says a thing than at least everyone has spoken once.

It's just the byproduct of "I don't have time to wait, need things now" phenomenon these days that the DF is adhering to.

You know, the most communication I've had to do via DF on a regular basis was the Halloween haunted house event. I legitimately enjoyed it even on my 10th run or so.