Why is ranked matchmaking so fucking scewered?

I'm talking about 2+2+1 vs 1+1+1+1+1 I'm talking about 3+2 vs 1+1+1+1+1 I'm talking about 5vs2+2+1 etc etc etc

I thought premades were supposed to mainly get matched vs premades? So why do I have to deal with this shit as a solo player?