You have the question backwards. Why does a game with so much RNG have a ranked system? In Beta?
How do you have a ranked system without Match History? How do you have a ranked system without advanced statistics recording?
I think that you're right. A lot of people agree with you. A few players have clearly already worked out the rules to maximize statistical advantage in this game.
A lot of people enjoy climbing. Riot was worried that people would be bored with TFT after a couple of months (didn't you get the survey?), and having a ranked system is a way of hooking people into committing.
It's probably not necessary. The game shares the qualities of games with random elements like backgammon and cards that makes them habit-forming and obsessive (and fun).
Competitive Backgammon exists... but I can't imagine watching it. And it's miles away from Professional Chess. The issue here is that Ranked TFT is equated with Ranked LoL.
TFT is a casual game, and it should remain that way.
But if we wanted make things a bit "more interesting"...
What would make more sense (to me), with this sort of game, is to have a kind of rewards system that had elements of a purse. Where you would win AND lose credits. Possibly even a betting system that let you set higher stakes. The size of the purse would be the indicator of a player's success with this game. It also might unlock flairs or cosmetics or other bonuses.
Maybe that really isn't all that different from what Ranked is anyway. Think of LP as chips.
...
You can enter with parties of 8. A tournament mode with a betting element would be interesting. I mean as part of the game... putting up golds before a round, to win more after. Maybe.