Why is there hero stacking ?

Blizzard why do you want this in the game ? You put tips during the loading screen "a well diverse team will raise your chances of winning!"..okay then why do I constantly get destroyed by teams of 3 D.Va's, torbjorn, etc.... I understand that if a team does this you can counter, but that doesn't make it right. To me, it allows douchebags to set the tone of the game, and really take away any enjoyment. There honestly isn't a single hero in the cast that I despise to the point where I never want them in the game, but I can honestly say you put two or more of any hero and Im ready to never play this game again.