So to ask, why keep RNG elements when you know they screw around with proper MOBA gameplay?
RNG isn't synonymous with "bad", and it has it's uses.
RNG is nifty for adding a level of excitement to a game, where you still have a small chance of winning big through chance.
RNG also smooths out a skill difference, allowing less skilled players to occasionally beat more skilled players.
In an environment with no chance at all, a player that's 1% more skilled than their peers would essentially win 100% of the time.
Which is bad for interest and a game's longevity.
Granted - it's essentially impossible to remove random factors like "I hope the enemy team isn't in Fog of War here" when running into an enemy team blind is possible through a vision war, and it's pretty difficult to gauge exactly where an opponent is going to pace to for the purposes of landing skillshots. Due to the factors involved, minion targeting/movement is essentially random as well, causing situations where you can get path-blocked or have a heroic minion take a hard left and catch a Nid spear with his face for you.
So Riot's done a decent job at incorporating random factors without really having to rely on RNG itself.
RNG is also Incorporated in the champion selection order in draft pick
In small amounts - RNG is ok even in a hyper-competitive game. You still want to give unskilled players a chance, and everyone starts the game as an unskilled player.
It's also a little silly to obliterate all random number generation when random factors do happen, especially when it comes to champion specific styles. One of the cool things about LoL is you can suck at strategy and still be a solid player by playing to reflexive plays, and you can suck at reflexive plays and be more focused on tactics or strategy. Some people do work well with simulated chance, others don't. In the case of RNG heavy champions like Shaco, GP, and Yi - you can easily not play those champions, and only run a 1/120-something chance of fighting each of them when fighting any particular champion.