No, it ISN'T.
His JOB is to do whatever is the best for the TEAM. NOT what's best for YOU or YOUR LANE. Just like it's YOUR JOB to do what's best for the TEAM - i.e. when he's not ganking for you to not die solo in lane, to not extend without vision and die to a gank, to not freely give up your tower, etc...
Ganking IS a key PART of the jungler's job. That said, WHEN they should gank and IF that gank should be for your lane are entirely different concepts.
If I'm playing a jungler that doesn't have great ganks pre level 6, and is a strong farm jungler, I probably shouldn't gank until I'm level 6. IF I am going to gank pre-6, it's going to be
- Against an opponent who is extended and doesn't have vision of my gank path - if you're pushing lane, don't expect me to gank your lane. If your opponent is freezing lane, don't expect me to gank. If your opponent is warding heavily, don't expect me to gank (I don't want to give away map pressure for no chance at a kill, and you should develop a gold lead since they're spending gold on wards and you're presumably not [pre 6]).
AND/OR
- Against an opponent who has no means of escape (e.g. you just told me you got your enemy Xerath's flash)
AND/OR
- For a laner who has their own means of CC (e.g. for my top lane Maokai)
Now, aside from all of that:
If you're say, Vladimir mid against Zed. Bot lane is Jinx+Leona against Kog'maw + Sona. Who do you think I'm going to gank? Obviously I'm going to camp bot lane as ganking to draw Zed's W is worth almost nothing, however even denying Kog'maw farm will make a huge difference in the game (and really, you'll need to give Jinx a couple kills so she can deny Kog'maw all laning phase in order to avoid him becoming a late game terror).
When Jungling, it's REALLY frustrating when a laner who is dying repeatedly in lane blames me for not ganking for them when I'm 4/0/5 and have been camping another lane - there's normally a REASON why I'm prioritizing another lane (in most cases EVERY one of these applies: it's the easier lane to gank, it's a more impactful lane [either the enemy or our teammate is a late game hypercarry], and the complaining laner is pushing [explains why he's dying, and I can't gank when they're already at tower]).
To the point of whether I should even gank or not:
- I may see a chance to counter-jungle, e.g. the enemy Blue buff should have just respawned and yet I saw the jungler in the opposite lane
- I'm establishing vision control near an objective (e.g. dragon/baron)
- I'm farming my own buffs to prevent them being stolen (or giving blue to mid, red to adc)
- I need to catch up on my own farm / I'm close to an item
- I'm too low health to gank without a high risk of dying against the laner / I'm OOM (out of mana)
- I don't have cooldowns that I need for the gank
- I don't want to show myself in your lane because it will allow a play against us elsewhere (e.g. show myself top and they get a fast dragon bottom, show myself bottom and their top laner can safely tower dive our top laner, etc...)
I'm sure there are many more reasons. Hopefully this at least explains why you're not always getting ganks when you want them.
More importantly though:
Not receiving a gank and even having a Jungler who OUTRIGHT SUCKS is NO EXCUSE FOR DYING IN LANE - ESPECIALLY REPEATEDLY.
It is NOT the jungler's fault you are dying in lane. The jungler NOT ganking didn't make you die. Your over-extending, your bad trades, your play of the matchup, etc... is what made you die. Once your behind, your refusal to play safely and wait to catch up is what is making you die. Your refusal to ward and have map awareness before extending is making you die.
If I'm in lane and another laner dies, I NEVER think "oh god this Jungler should have ganked for him". I think "lets see how close he was to getting the kill (was it a small misjudgment)" and if it wasn't close "wow this laner sucks".
More notably, it can actually be the laner's fault when the jungler dies. Either from not going in when he ganks, or more importantly not noticing he's skirmishing with the enemy jungler in the river and letting your lane opponent go change a winning fight for your jungler in to a losing 2v1 when you don't also go to the fight (I literally had this in my last game, winning 1v1 against Vi in top-mid river after getting an assist ganking Teemo mid, my top laner Kha'zix had pushed to tower, I pinged assistance even though I was going to win the fight, and somehow the enemy top laner Wukong came to assist Vi while neither my top nor mid Cho'gath came to help me [admittedly Cho was low after I ganked, but Kha' should have been there before Wukong]).