For everyone saying it is a group effort and that it can be a healer or DPS issue as well are correct.
However it is easier to see tanks and healers mess up more then DPS can. Sometimes under performing DPS do not even know they are the issue and see someone die, healer blame (and I got unfair blame myself over that) I had to say " ohh nonono people are not dying because of my healing, people are dying because they keepstanding in stuff you need to dodge and getting 5 stack. I can't heal 1 hit kos" This was a comment givin to me when I tried to suggest to this one BLM who was underpforming since I can see cast bars and know what they are doing for rotations. Basically they where using blizzard IV and blizzard III too much and I told them that, and they said they where doing fine but it is my fault when people die to 5+ stacks.
So yes healers and tanks can be passed for blame for no reason because it is easier to see issues with regarding healing and tank, and can be misleading as a surface issue.
edit:
How fitting, I was doing leveling roulette so i can get 18 clusters and got some kind of troll BLM in odd gear. That's okay I first thought as long as they try it will be np. Well I noticed things where dying too slow so I was trying to figure out why. (fire I and fire II can look alike animation wise depending what is going on) So as I was trying do my thing with heal and dps it was a bit hard to focus on what the BLM was casting but I knew something was off.
This ended up being proven right when the BLM pulled hate from the tank, usually when that happens it has to do with over dps off single rotation. So that is what they did for that pull casting fire I spam, fire III procs etc. But on the boss on add part they did a proper aoe rotation and I thought maybe they noticed the mistake. Well next pull was single target again when I commented, "fire II spam 3+ smonters plase then flare, convert flare" next line "aoe damage is low atm" but no one said anything back except the other dps doing "?"
After a wipe I said their name with saying "Aoe rotation please" then the DPS said "BLM is doing fine" I said we wiped because of it, because of that + a pull I was not used too so I was unsure how time healing spells and the damage taken from it (faster then I expected) So then I was kick abused out of the vault, only to enter a new vault DF for leveling weekly quest for clusters.
I hate vault so much, along with a tank that can't see the issue with a DPS pulling hate because they are fully going out single target rotation with a grouping of 5????
So yes people are really blind to DPS issues and blame others because it is easier to see. In people's minds, tank not holding hate = tank fault, people die = healer fault while not looking why that happened. (singe target DPSing or stuff not being killed fast enough)