I've won 32 of my 35 games since the reset. I might not be the best but I consider myself pretty decent. I'm about 200 ranks below you, but you play with a premade team so drawing too strong conclusions from that might not be a good idea.
What I can't control is the random partners that I get and since good PUGgers avoid premades those random partners tend to be complete noobs when going against a premade. Playing a Sedna helps somewhat as I can babysit a noob, but I can't win by my lonesome.
32 from 35 in a 1v1 game like Starcraft would be a godly record, unfortunately stats don't mean anything in DG because it's reliant on passable teammates, even if you can solowin to a degree against newbies. Rank is also pretty meaningless, since it's based on experience which doesn't seem to be a measure of...anything much.
I have no reason to believe I'm a better player than you despite having a higher rank (you're probably better than me, I've been playing DoW2 lately), and the shoddy stats tracking is only further hurting the estimations as to who is good and who isn't.
I stand by my assessment that we just need people to be better at the game, and a team automatch option.
A big chunk of my games that you would class as "Premade" (and someone had the nerve to whine at me in game for being a stacker while I was doing this) were actually me on the mic teaching a friend how to play. A fair portion of the games I play with Verspunken we try out more left-field tactics that invariably are less effective than just health stacking and buying creeps, so it's not like we are making game after game and stomping through all opposition unopposed. If people like Ryanheart and Stacked Actor join our games its generally a decent fight, the problem is that there just aren't enough people at that level of competancy and a single poor player can ruin the game for all of us.
To answer your OP: They don't play each other because it's extra effort to get a game going. A single random joining so one of your teammates can't get in creates issues. Hosts playing with fsk stupid game settings causes issues. People wanting no doubles/no minion erebus/no people with 3 arms cause issues. Host disconnect problems cause trust issues. People wanting to play Exile because it's pretty cause issues.
To avoid all the hassle, most premades just make their own game. There's not even any guaruntee that a pre vs pre game would be anytihng other than a walkover anyway, many premade teams I've seen have been worse than randoms.