Because EVERY lane can blame them and people just accept it since it's not them. Jungler gets ganked constantly by lanes? Blame jungler for not ganking. Your lane is losing so bad you never have HP to get a proper gank? Junglers fault for not ganking. Push under enemy tower all day then die? Junglers fault for not ganking a wining lane.
Add to that if you're playing your lane poorly "enemy jg keeps ganking, nothing from ours" because enemy lane is positioning well and yours is positioning badly allowing for it. Junglers fault.
Also, for some reason laners are under the impression a JG can successfully gank into a winning opponent despite having no HP to help the jungler when junglers are designed to be weaker than laners.
I challenge any jungler to /fullmute their games and NOT have their win rate increase.
Hell I've had a lane thank me twice for ganks early game, then blame me late game for not even going to their lane once...
Here's a harsh truth, junglers do better when they ignore allied requests. I even watched some challenger JG streams and a few have said their largest path to improvement was learning to ignore laners trying to tell them what to do.
It's easier for people to blame others than themselves is what it boils down to. JG is the only one in range for most to blame.