Their gross income is in the billions, first of all.
But their overhead also comes out of that. What people too easily forget is that if this game stops being profitable, Riot will abandon it. I'm not asking for people to let them get away with murder; I'm asking you to allow capitalism to happen.
You don't like it when people look at your paycheck and say, "oh well, they make plenty so I can just take some of it and it's not important." That's not right. No matter the amount, no matter the percentage. It's faulty logic.
Those billions of dollars go to many Rioters who, just like any of us, have to pay bills and eat and support their families. If you're uncomfortable with them making large sums of money, then don't play the game. Pretty simple.
Just remember that all that money isn't going straight into anyone's pocket. Some of it is distributed into thousands of pockets (I'm not actually sure how many people Riot employ overall), and that's good. That's what gives them the incentive to work on the game.