How can that be? It sounds easy right? Just beat on lifeless mobs and come out of the fog of war when the other team overextends and win 2v1 situations or 2v3 situations right?
Nope. Essentially the jungler is the leader of the team during early game and mid game.
At a higher level of play, people should hardly be dieing in lane unlike alot of low ELO players tend to do alot. At low ELO people dont understand their matchups, and they'll waste all their mana and health and summoners trying to chase eachother down into the turret, while barely last hitting.
However when both teams actually know how to play, there is almost NO early game action. The side of the lane that has the upperhand in lane will establish their dominance without overextending by thinking they should be killing instead of last hitting and preventing the other from last hitting, while the person at a disadvantage from the laning perspective will just last hit under their tower.
When this happens, just about the only thing that incites early game action is the jungler. The jungler should know not only their own jungler vs. jungler matchup, but every single other lanes matchup, they should know their own common paths, backup/recovery paths, the should know the junglers most common paths, and their recoveries, figure out what lanes the other jungler will gank so they can counter gank/and/or counter jungle, figure out what lanes are the most efficient to gank, figure out where the other jungler will be low, make sure you are not low at a point easily taken advantage of, and if you are low at a point that is unavoidable, to make sure your team is there to protect you, all while controlling dragon and playing whack-a-mole with your small camps.
Not only this, certain junglers are built to give your team an early game advantage, and certain junglers are built to snowball out of control and carry. Saying this, because of his interaction with the other lanes early/mid game and not just the synergy late game, the jungler has the most pressure and importance in his pick and choice of champion.
In short, the jungler has to know EVERYTHING and EVERY ASPECT of early game, so(as the most mobile AND hidden champion on the map) he can capitalize on advantages and coordinate/synergize/protect against your own disadvantages.
ANY time you are countered in some way on the other team, you require the coordination of your own team comp to beat that counter. Every counter in this game comes at a certain price.
Countering your jungle costs the other jungler in experience from all his travelling back and forth...However, if you are not able to capitalize on this and gank him while
he is in your jungle, then he wins the exchange...it's a high risk, high reward situation. Capitalize on the risk, or he earns the reward.