Why cant I block getting matched with/against parties?

Because matchmaking is currently no where near valid.

Why people complain so much about parties?

Being in a party does not imply that they will have for sure better communication, or that they will be good at all. Statistically, someone in the party is bound to be either above or below the average MRR of the game. If they are below, they could very likely drag the party down (like someone lower ranked solo can in your own team). If they are above, even them have bad days, or pick a hero that they aren't as familiar with (same as a solo player)

Just last night I was with a 2-stack + 2 solo + me, vs a 4-stack + solo (unranked of course) and we won with some ease. The solo used AA, which doesn't need a lot of team coordination either (point Ice Blast at the fight, fire).

Don't people ever see when they win if the enemy was in a stack? Because for sure they will remember to check if they lose.