Thx Babagahnoosh, I always loved the lore of the game as well as stories and fanfics.
Originally Posted by Babagahnoosh
Ezreal couldn’t argue with that; the most important thing now for him was to aid his summoner. Ezreal followed quickly behind Ryze, staring intently ahead. He became a League champion to make a difference; learning the secrets of summoning was secondary, at least for now. Worrying about the minutia of summoning wouldn’t help him win or lose this battle. There would be time to figure everything out later.
I bolded the key part, according to Ezreal's lore he's a champion not by choice, but because its just a price to pay for the power he gets from his necklace.
This is the kind of stuff you get from lore nerds :P