That however is non canon nowadays and there is no reason at all.
Really? Because I can think of a couple at the top of my hat:
to avoid massive wars
Which essentially stopped any potential for lore development or even a sense of threat. People didn't need to be afraid of the Void consuming the whole planet, Noxus starting another war or the Shadow Isles depopulating entire regions, because the summoners were basically omnipotent and benevolent (for whatever reason).
semi mind control the creatures summoned from other worlds
It already was getting silly with Void monsters that could eat worlds and sentient mountains, but if the Institue existed now I would seriously question how they managed to trap and control the manifestation of death, a starforger and a cosmic guardian.
Everything else was constructed by the summoners as part of the arena, such as the minions and even Baron
I always wondered why they never used all those resources to become the undisputing gods of Runeterra. Who is gonna stop an army consiting of thousands of summoned barons?
They really did kill each other, but summoners have the power to bring them back to life.
Which only served to make them seem more powerfull and death less meaningful.
Instead of making the game feel like part of the lore it only served to make the lore feel more game-y, at least for me. It also never explained mirror matchups, the sheer quantity of games/summoners and why champions aren't locked in some form or other (would you seriously kill your own sister over and over again, even if she won't permanently die?).
Honestly the headcanon that it still is the Institute of War, but removed from Runeterra (either by time or space), makes the most sense for the gameplay and doesn't mess with the lore.
<3 
but they also be like 
