Junglers farm because there's no real gain to ganking all the time.
Say, I'm vi. Her best gank is her ulti to lock down after a q (provided ADC is in formation to follow up) and do some damage to anyone in the ensuing chaos. Then leave.
While ults down, I can probablu gank mid if they have hard/reliable cc. But hey, that 175 gold I got from that assist. That's not doin much for me.
You have minions that literally walk to you and die. You get exp from just being there. We gotta go kill jungle creeps, ward dragon, secure dragon, cover lanes, covrt our own jungle, get vision crabs, counter jungle, counter gank (to keep their jungler off you) all while having to babysit the team and deal with their qq whie they flame you.
I vented a bit. I apologize. Junglers have more things to do than anyone else.