It's because there are too many variables to this game. Rito cannot balance it even if they tried their hardest ( they don't though)
If you take 4 strong mechanical players and add a strong tactical player and the 4 mechanical players are actually listening to the tactical player.. then you are nigh unbeatable.
If you take 4 strong tactical players but poor mechanical players and add in a strong mechanical player, you will most likely lose. Your shot calling will be shit and your team fights will be a mess. Your objective control will suffer, your jungler will most likely AFK masturbate and it's not gonna be fun.
Take 5 of either kind and it's gonna suck as well. 5 strong mechanical players will lose the game with more kills than the other team and so on.
These examples are from people who actually have SOME strength in this game.
Let's put together a Team Builder team.
1 Player who is "first time X" but is not gonna play safe at all. He will feed and he doesn't care, he is just there to learn the mechanics of the champion.
1 Player who is an AFK farmer who always wants to be the late game hero who emerges with stacks or stolen easy kills to save the day, except he won't.
1 Player who will sacrifice himself all game, even when nobody was in any real danger. He will go 0-18-9 and it doesn't matter cause he was the support.
1 Player who just shat himself when he lost that final smidge of faith in humanity upon seeing his team move around the rift.
1 Player who will complain about not getting any free kills, then her power spikes and she will proceed to get every last kill of the second half of the game.