Because I'm just a casual player.
I've used voicechat and have been part of premades, but I got turned off for a few reasons. The first reason is that it seemed like, unless we were doing an inter-clan fight, we always fought low-skilled PUGs who didn't know any better and got stomped. That wasn't that fun. The second reason is personal: judging from the voices I heard on Vent, I am an older player in the Demigod community (34). I have a wife and a demanding career, and while it's fine to play a couple of games of Demigod in the evening, I don't want to put a headset on and isolate myself from the rest of the house. The third reason is my experience was that the people who use Vent were too hard-core for my tastes and cared too much about winning. I couldn't relate, and, I had no desire to subject to myself to hearing a teenage basement dweller who yelled and whined like his balls were being cut off just because a flag wasn't locked within 5 seconds of when someone demanded it, or because a heal didn't come in at the right time.
These are just my personal experiences. I'm sure there are pleasant premades out there somewhere, who don't care about losing.
I have no doubt that only dedicated premades can make full use of the strategic potential of the game. For example, playing exclusively as a PUG definitely skews you to take Erebus, Sedna, UB or Oak because you need to be as self-sufficient as you can, and can't count on your teammates supporting you well. But it's still fun playing in all PUG games, as long as you don't care about losing.
By the way, PUGs are not blameless here. I can't tell the number of times I've been in games where PUG players tried to stack teams based on win/loss records. I can understand the motivation- no one wants to play alongside a noob- but it's still embarassing. The times when I've asked players to mix up teams, no one has ever taken me up.
I picked up Demigod because I thought it had space for casual players as well as hardcore players. Wasn't that how it was marketed? Maybe the real problem isn't PUGs vs. premades, it's that the community is too small and the interface isn't set up effectively enough to segregate hardcore vs. casual players. (By casual, I do not mean Noob players, who don't know what they're doing. I mean PUG players of moderate skill who know the basics but don't have time/inclination to do anything outside the game lobby).