Can't read peoples mind or anything, but I'm pretty confident the nr. 1 reason people premade with 4+ players is to steamroll the other teams, ganking up those kills. Sure, on paper it's fully possible to beat a veteran premade team, I agree, and have done so myself, but for the majority of the times, playing with randoms vs premades (assuming premades are skilled) is a losing battle no matter how much of a skilled tactical genius you are. If people won't listen to you, tough luck.

Back in 3.x when queues were miserable it could become extremely frustrating when you'd play multiple days juggling between premades 24 vs 24, in an ever losing battle. These days it's not really an issue, considering the frustration factor, due to freelance and much better queues.

Imo, frontline should limit premades to light party. But I'm no nazi about it.