Teamwork is fairly difficult to accomplish in a Team Deathmatch mode where the vast majority of players don't know each other. Objective based modes tend to force teamwork unless you really like to lose, so hopefully when objective modes make it in game, everyone'll start seeing more teamwork.
That being said, the primary reason for lack of teamwork as far is I can see is that people don't know each other. New players are often timid and don't speak up/ask for help, and that goes for seasoned players as well. While it's true that in-game chat is slow and clunky, it can still serve as a basic organization tool; all you have to do is speak up and listen, and just not assume that you know best.
I played a match yesterday on Sandblasted that had one of the most organized starts I've ever been a part of. It was a fairly good size match, maybe 14-18 people at the start. At the very beginning, nearly everyone on my team grabbed a Light Mech, and we all moved as a single blob up the north dunes. The other team had fanned out as single units, and we just pounced on them one by one, just bombarding them with bodies. We killed other Light Mechs, tanks, and even Aircraft just by teamwork, and seized the majority of the map. This trend continued through the entire game, with our team barricading the other team near their spawn (to be clear, there was no spawn camping); I don't think we were ever pushed back pass the mid point of the map, and we won by a wide margin.