I can give you the top two reasons it's basically copied:
- It makes money.
- See 1.
NHL: https://www.nhl.com/standings/2017/league
NFL: https://www.nfl.com/standings
EPL: https://www.premierleague.com/tables
Oh, here's my favorite, LCS:
http://www.lolesports.com/en_US/na-lcs/na_2017_summer/standings/regular_season
They're all based off win/loss. They're all transparent. They're all competitive. They're all ranked properly. They all have larger fan bases than League.
The only way a system like this works for random individuals forming a team is to have the entire pool of players competing to be rank 1, everyone on a level playing field. You, me, Aphromoo, that dude in Bronze 5 playing tank fiddlesticks jungle. How do you matchmake? Because not everyone is going to have a chance to play everyone else like they do in the above leagues, it needs to be based off something. Win/Loss ratio. If you're playing at 50% win/loss, you get matched with others at 50% win/loss. If you're playing at 70% win/loss, you're matched against others at 70% win loss. This is the ONLY way you'll ever have a true competitive environment in Solo Q. I'd even go as far as disallowing Duo and making it true Solo Q and bring back Team 5 v 5s for the more serious that want to play together.
MMR and LP only serve to fill Riots pockets, not to actually rank your skill. There's a number of income sources based off this system, some blatant, some not so much. They run a business, the game is actually just a side hustle.