Tank dodging

[TouchandFeel]

I don't understand ...

Some pulls have priority targets that need to be single-targeted and taken down separately from the group, so the dps wouldn't be AoEing in those situations. A somewhat rare situation but it does occur.

Another thing that is of consideration is that you often have enemies that do AoEs targeted at the tank and if you are moving around, where those AoEs will affect is less predictable and could result in your party getting hit by or having to unnecessarily run away from an AoE because you spun the mob and now the whole party is in the middle of a large cone AoE.

The most common consideration though is that in large pulls you want to keep the mob group packed in together as tightly as possible to help make sure that they will all be within the range of the AoEs your party is pumping out. The more you move or run around, the more the mobs will shift and move around which can result in them moving just out of range of the AoE's.

Basically it's best practice as a tank to try to minimize the amount of movement and adjustment that the party has to make. If you are running around, the mob pack will also be moving and this forces your party to move and adjust.
Also, as it has already been pointed out. Running around doesn't stop auto-attacks from hitting you, so it provides basically no benefit to do and only creates extra burden on your party by making them have to adjust for your unnecessary movements.