Question about Roles and Champ Design

Ok, I have a question about game design of League. It's a MOBA and RTS, meaning that strategy is key. Thinking and game knowledge are higher valued than pure mechanics and bloodbaths. So that brings me to my question:

Why are assassins a class?

Assassins are the definition of feast or feed, they rely on instagibbing with mechanics and the extent of the strategy involved is just knowing when the enemy is done with CC.

And it's not like they're well supported by the community. Look at all the champions players like to QQ about:

So, why is it a thing? All answers welcomed, subject to argument. And please, no Riot bashing or illogical rage.