Because the game tries to match you with whomever is searching for a game at the same time. It would work better if there were more players playing.
If your rank with random 4v4s is low you're kinda screwed. It's pretty much entirely random who is going to have the worse teammates.
Back when I played 4v4 randoms a bit for fun, I was playing soviets and doing retarded spam strats to mess around. Tanked my elo there. It's pretty much unplayable now, everyone it matches me with is god-awful. It would require insane amounts of effort to raise my elo to get out of those ranks. It's entirely dependent on which team gets lucky and has the least bad players.
Moral of the story is start out good 
This is exactly what happened, in the first 10 games (these are with a smurf account) in Soviet and OKW, no one dropped, no one was AFK, everyone in the match was a good player, the result of which was high rank after the first 10 matches (Soviet had 100% win rate, OKW had 70% win rate). When I did this with OST, the first 4 games had AFK players, probably 6-7 of them had people on my team that dropped after 1-2 minutes, the result of which was a 10% win rate and rank 2500 or so....now if I play Soviet or OKW in 4v4, they are mostly enjoyable, but every single game with OST is anal cancer, and I mostly lose the matches, further sinking rank, making them progressively more and more cancerous.