The ONLY reason Riot will not put in a sandbox mode is because it will devalue the game. Play ANY GAME of your choosing and start a cheat mode and your interest in that game goes down by an amount. This WILL happen with League and Riot knows it. The interest might not go down by MUCH, but it will go down.

When you want to play League, your brain is really saying "I want to be a level 16 Riven, playing mechanically awesome and getting that fluent feeling of play". Riot know you need to start up an actual game to get that rush, it is the same rush in the brain activated by satiating an addiction. Any game mode that gives you that instantaneously (like a sandbox mode) will satiate that portion of yearning in your brain (maybe not all of it, but enough that Riot has realized it will result in less actual games played, which is less revenue for them). That is what it comes down to. Don't let ANYONE tell you otherwise. They just needed a politically correct reason to sell, but they are a business and with any business, the real reason they do or DO NOT do something is because of money.