Tanks can't keep aggro.

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Bad players in a MMORPG? Bad players failing at the single easiest role in the game? Nobody could ever have possibly seen this coming.

While tanking is usually the easier role compared to healing (I'd put DPS and Tanking on the same level in terms of difficulty), it is also one of the most difficult roles depending on gear discrepancy, confidence in ability, and numerous other variables. (This is, of course, assuming the tank isn't actually "bad", not that there aren't tanks that don't know what they're doing).

When you have a slightly-moderately undergeared tank in a low level roulette or even a level 60 roulette doing literally everything possible to keep hate and still losing it to the synced down DPS and/or the healer (usually pre 30 for mrd/war, pre 40 for gld/pld for tank stances). While the game does give you all the equips you need through quests, not everyone does all the side quests nor are any of the quest given pieces of gear high quality pre heavensward.

And in the case of level 60 content; i150-170 weapons on a tank will eventually be unable to keep up with say a DRG or MNK going all out with their shiny new anima weapon if said tank is managing multiple targets due to the BLM spamming AoEs on a group with their shiny new i210 weapon. I say this from experience on my i175ish DRK on my alt (Tanith Aurion), I never lost hate, but it was difficult maintaining it on groups even on smaller pulls. The Monk in that group was also usually hanging around 80% of the enmity on bosses (in the Arboretum) too.