Season 7 in a nutshell.

I really think league players have never played a single MMO in their lives. League of legends is a moba, mobas are inspired by a what if scenario if an MMO purely dedicated to PVP instead of PVE. So because of this, the basic logics of a MMO will translate into a MOBA, for PVE came out before PVP.

And in the standard PVE raid comp your team party often consisted of.

1-2 suppports 1-2tanks 1- dps mage 1-Melle carry

and oh look that system is in league, dota, over watch, it's just mathmatically the best stratagy in most cases.

2 Front liners go in and soak damage While helps them rotate agro while doing big chunks of damage in the mean time or is shooting safely from far away while doing the majority of the damage than lastly you have as team white mage to heal whoever is getting to low for comfort

It's also the easiest of comps to pull off, which is why they are the most played, low risk high reward.

Well, as long as ADCs are broken as they are, playing anything but a Tank means you die in 2-3 Autohits.