Because it isn't simple. Range, no matter how weak, is an advantage. But if you just make the ranged champions shitty it feels shitty playing them. Mid/Late game it isn't much of a problem. Only in laning where enemy and you are forced to stand face to face for 10-20 minutes. Them out ranging you there is huge, so instead they made ranged champions immobile and/or shit early game so they can scale to relevance late game. But people bitched about diversity so Quinn and Graves, Gnar and so forth came to be. Now it is a huge problem.
The solutions are not simple. We could go back to all ranged champions are shitty and vulnerable early, which fixes it but limits design and means they have to have an excessively rewarding late game.
We could buff the shit out of Doran's shield vs ranged, but... that is the same as above except it now forces anybody facing a ranged champion to buy a Doran's shield, even if it is not optimal on them at all.
We could bar them from playing in certain lanes, but that does what everyone hates, enforces the meta.
We could make melee champions super OP early game, but they all have to fall off as hard as Pantheon late game which feels terrible.
There is no simple solution to fixing range vs melee, or Riot would have done it already. Most of the solutions either limit gameplay, tilt general balance or require a new engine and overall game design. Riot isn't just sitting on their ass, it is a hard problem to crack, or else we would still use swords vs guns in real life.