A few reasons in my opinion:

1) makes it more difficult for mob control if the tank doesn't have initial aggro as mobs will not converge on him/her.

2) dps take more damage than tanks which means extra healing which means healers get aggro much sooner and more of it before a tank can control the mobs

3) less controllable situation overall. The tank can't position appropriately ASAP to maximize said aoe dps you claim makes it easier to do if a dps pulls.

This is from a healer perspective. Pulls go smoother 99% of the time when a tank pulls first.