There's a whole world of difference between 'playing with friends' and forming voice-coordinated teams with the sole intention of controlling the game to the point where opposing teams have to form a silent alliance to focus them down, with one random team practically forfeiting the match for the sake of the other.
I play with friends. We queue together to be on the same team, but aside from that we play separately. My support/coordination with my friends is no different to that of random players on the team. Our competition remains between each other, comparing scores at the end. These premade teams are totally different, taking heavy advantage of the fact that they have a pre-arranged team which many players may not have (PvP isn't exactly a highly popular part of the game and matters like this contribute to that) and using voice chat to reach a level of coordination which cannot be replicated within a random team. It gives such an immense advantage that sometimes there is practically nothing that can be done to overcome this setup.
You are heavily playing the problem down when you say "they're not ruining the game for other people". Matches are supposed to be as balanced as possible, with each teams having a fairly equal chance of success. Yes, other factors come into play which skew this slightly (class imbalances and team composition, RNG, player experience etc.) but in a regular match each team has a relatively equal chance of winning. In matches with a premade, this 'balance' is heavily disrupted. Sometimes, depending on the premade, it can still be worked through with good luck and a strong team. But in some premade setups, this just isn't possible.
There shouldn't be this level of imbalance. It shouldn't be the case that objectives have to be voluntarily 'given away' to the random team in second place just to keep them in the game while you both focus down the premade's alliance. Games become predictable and dull. More and more players will be turned away from PvP. Why bother when wins are almost guaranteed to go to whichever team has the premade in?
Can we do it? Yes, I suppose if I wanted to I could always organize my own premade with the friends I play with and repeat exactly the same as them. It doesn't take any kind of skill.
Would I want to? No, because I enjoy PvP, I like fair competition, and I like to see more new players joining us in these games.