Nerfs/Balancing Items Vs. Champions

With the recent discussion about junglers getting thrown around about how everyone only sees VI J4 and Rek in the jungle and the youtube video straight up RIPPING into riots decisions on patch 5.4 I thought of a few things which i wanted to bring up to you guys/maybe get some feedback or explanations. Jarvan and VI have always been strong jungle picks but now that the hunters machete gives them both a mini which benefits from the jungle. This makes them better because now most jungle champs which already had bruiser kits now have tankiness, Attack Damage and penetration, and the ability to smite champions which gives them even more ability to gank. This wasnt so easy with them back when all we had was either golem or wraith spirit stones. Which leads me to my questions: Why does riot nerf/Balance champions, which are going to be permanently in the game, instead of just going all-out on the items which as we all know are going to come and go? This hit me because my friend noted that some champions(i forgot which ones because i never play mid) were nerfed years ago because DeathFire was an item which, when combined with their kit, made them super nukes. He went on to say that some of them deserve to get their stats back since its no longer in the game. Also, RIP Deathfire, I'm a little late to the party there but still...

TL;DR: Why does riot nerf/balance champs like: Who we know are going to be in the game forever Instead of Harder Nerfing/Balancing on: Which could come and go next year.