We need something to do once we've hit the max level and built a few champs we like.

As of now there is nothing to strive for once we've capped to keep us playing the game. Yes, we can buy and build all of the champs but that's probably gonna be a grind against bots with very few competitive matches sprinkled in. Topping the leaderboards is basically who can play the most and stomping out bots quickly with a group so thats not an interesting goal.

Plus us stomping out new players and seemingly (In their eyes) not taking any damage while instamelting them probably isn't good for player retention.

TL; DR

Players at endgame need something to do besides grind more.