This gets asked a lot, but I understand why the devs did it.
Most of the time when gamers are playing, they are playing by themselves. This may not be the case for you, but it's the case for others.
If they allowed pre-made parties, the people who play by themselves will think "Why bother queueing in? I might get put up against a premade" and choose not to play.
Imo this is actually better for the longevity of the game. People who play with friends almost always move on anyway, because there's always one or two friends in the group who move on first and sooner or later you're back to spending most of your time solo.
So basically, this design decision was to help soloqueuers over premade players. And I think this is a right choice.
But I'm biased because I'm a soloqueuer most of the time.

All that being said: This game mode is going to be dead in the water if they don't deal with the glaring balance problems.