Some people will blame whoever they think is doing worse (or as bad) as them.
I had a ranked game where our 0/13/0 Poppy Top blame their performance on our 5/7/6 Pantheon ADC's CSing. Not his KDA, not getting his enemy laner fed, his ability to CS against a ranged opponent....
As for Jungler's specifically, I think some get blame because it feels like they're doing nothing. I see way too many Junglers trying to go Sated but who have no idea how to stack it, avoiding Scuttle Crab and Dragons like the plague and generally being useless in a game that was looking upside down before they even bought their Devourer. The problem is made all the worse when the enemy Jungler is visibly doing something.
Sometimes it's a problem with tunnel vision, but sometimes it's a problem of different expectations from different Junglers. For Supports, if they ward and either set up kills or deny kills, they're doing their job. No one besides the ADC expects help from the Support early either. Contrast that to Junglers, where some Junglers will gank your lane 7 times in the first 5 minutes of the game, some will pressure objectives like a man possessed, others will AFK farm the jungle until the game is lost, some provide damage, some provide utility, etc.
While you as the Jungler expect someone to know you're not ganking until 6, I think some people expect you to know that the matchup they're in is hard and not going well, and for you to help them. Not really saying which is right or wrong, but I think as easily as you can defend the Jungler for not ganking you could defend the laner needing the gank.