It doesn't matter how good you are at jungling. You will always be 2 levels behind everyone else on the map aside from support. You have to spend all your starting gold on jungle items, clear the jungle quickly without dying via invasion, go b to get an item that will help gank, go back into the jungle to try gaining enough xp to make an impact in team fights while hoping your jungle wasn't invaded/your whole team doesn't die on you, and then give the occasional gank with, at best, a 50% success rate, in which a failure is just another crippling blow to an already underfed team mate.
I can spend most of a game in the jungle as Warwick and kill every neutral unit in there n record time and whenever they respawn and I will still be behind everyone on my team.
And how often does a Jungler catch hell for killing an enemy champion? I've been in PvP where the Jungler has been reduced to apologizing for killing enemies when they were trying to get me kills. Why? I'm getting my endless supply of XP and gold from wave after wave of creeps.
People in lanes are in the presidential suite when it comes to getting fed and pampered. The jungler is the traveling gypsy whose presence is loathed if they don't have a 100% success rate!
Riot has turned the Jungler into a bench warmer. He practices off screen to prepare for a game turning play that may never come and gets sent in when someone on the team needs a refresher. At this point, the only viable junglers are ones with insane sustain or massive CC. Assassin jungle? Yeah, right. Not anymore. They're better off dominating mid lane where the most XP comes in.
Additional: And when laning ends, ANYONE on your team can jungle. I recently watched a ranked draft team where a Twitch took dragon at every given opportunity! So then the Jungler becomes a commodity whose biggest impact in the game was staying off lane so the rest of the team could get uber fed.