Here is why: communication is key, and a premade will always communicate, while a random PUG team will rarely.
Half of the games I'm in with PUG's it's me filling up the entire chatbox coordinating my team. It's hopeless: just one in 20 random noobs can freakin TALK or READ. SO no it's not me, I'm really coordinating talking, and no I don't suck.
Second is that as a premade you can be abysmal players but you aren't retards, if someone feeds the whole time, his mate will teach him how to die less. In a PUG the feeder won't listen.
Seriously, why is it that the majority of the randoms I play with, are deaf, blind, mute, hyperdyslexic, ignorant, rude, mentally handicapped, or just fuking retarded?
SERIOUSLY? WHY?