Thief isnt a melee class. it can simply melee as an option.
The point is that melee combat in general is a death-trap and it's much more viable for the classes which are even tooled specifically for high-dps output melee to simply stay at range. Viz. it is wiser for any conservative or tactical player to always stay at range, doing damage with a gun or shortbow. Viz. melee is quite useless in comparison; there is no need or convincing reason to use it (other than the fact you simply get bored of spamming the same 2/3 shortbow skills, and want some self-imposed variation and challenge).