Why do non-caster DPS do this...?

[kaynide]

Depends on a lot. Switching is more about not wasting actions.
If you are doing AoE on a higher hp mob, it is less likely the mob will die on you and waste an action. Many AoE do more damage to their primary target.
If the mob group is about done, it might be better dps to hit with single target. Typically 3 or more targets = AoE. 1 or 2 = single target. (Ymmv depending on job/ability)
Also also, DoTs, non gcd abilities or tank hate AoE can easily finish off stragglers.