This is so obvious in OW and HOTS!
The games are decided before they begin by setting up massive skill stacks or poor composition. I have 895 games played and 446 wins. Pretty much exactly 50% to within a game!
I know the Blizzdrones will say this is the sign of a working mmr, but a) when literally 80% of my games are total stomps and b) the win streaks and loss streaks are a pattern like clock work. The funny thing is that I had the same experince with HS and OW. I genuinely maintain that Blizzard have never been able to setup a genuine matchmaking system, so enforce a 50% winrate by artificially setting up games.