See, here's the thing, Riot's a game company that runs on a F2P model. This means that they really don't care about whether or not you have fun or even if you like the game, so long as you (or anyone else) throws money at their cash grab of a shop. So, essentially, they will do whatever it takes to get you to pay through the nose for champs, skins and so on.
Why would they make so many champs that have very bland, predictable mechanics? Easy. It makes them money. Think about it, how often are the "harder" champs (Azir, Nidalee, Ekko, Jayce and so on) actually played? Compared to champs who do nothing but click (Vayne, Annie, Fizz, Riven, etc), they may as well be Yorick. People don't like to put in effort for anything these days, and that applies doubly so to video games -- if you made people actually work and put in real effort / skill to win, they just wouldn't play the game. Clicking is familiar, and clicking is easy. So instead of learning a whole new kit's worth of interesting mechanics, you have rehashes of other kits you're already familiar with, and the use of mechanics you're already familiar with (they may have one or two new, but minor, mechanics (their passive and W's zone)) -- this makes Riot more money, because people are much more likely to pay if they feel like they can win.
So, why so many auto attack based champs, and why no complex or caster champs? Well, that's because money is the ruler of the universe, and Riot is going to do anything they can to get every single cent out of you -- and no one would pay through the nose for a hard champ.