I also hate playing with premades because it promotes toxicity
Sure, it's annoying when my team is decimated by a premade or when you find yourself getting carried by another, but why aren't people talking about how badly the conversations play out?
4 man premades on my team always gang up on me in the chat box. A single mistake, a single missed skillshot, even a missed minion if they're picky enough, and I have four allies (or so they're called) coming at my neck after one of their perfectionists call me out. It's tiring. We're already bombarded by frustrated people who want us to ban this champion, pick that champion, build these items, win this lane, kill that enemy, carry these fights, etc. but now us solo players have to endure abuse from not only one teammate who expects everything from you, but three others who will back each other up no matter the circumstances.
I never enjoy interacting with premade groups of people, even if we do win.