Why do people always rage at their jungler?

Because it's the easiest lane to fuck up royal in? And if you do you can divebomb the entire team. That being said try playing support some time.

Who else are solo lanes gonna rage at?

Jungle is the default scapegoat of any given team. If you sign up for jungle, you sign up for the flaming hot sauce, extra hot.

Not all teams will flame the jungle though, it's the default setting.

Because the jungle is a role, and that role is extremely dynamic. Laners constantly need to worry about the enemy jungler and depend on their own jungler. But either way, that person gets the blame.

Laner is losing in lane: Worthless jungler not helping me.

Enemy jungler is pressuring more: Worthless jungler farming while we are getting raped.

Jungler causing pressure around the map, but one lane is losing while you're elsewhere: Worthless jungler, HELP ME.

Laners feel like the junglers role is to win every lane, destroy the enemy jungler, have perfect control over dragon and baron, perfectly time smite and back times, etc. Alot of people think jungle is all about mindlessly farming AI camps, and they pin the blame on that individual for their own mistakes.