What sense would it make for a Dracthyr warrior to be covered in plate armor and swinging a sword, while a Dracthyr Evoker is pretty much naked and just using claws? Evoker players are going to start asking for the weapons and armor to show up in the Evoker class as well. On the flip side, non Evokers are going to start asking for abilities in the other classes that allow them to use their wings, tails, and breath. Now you have a development mess where no one is happy.
Again, best not to bother and keep things the way they are.
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Dig through Wowpedia, it's in there. You can even look at WoW-based Worgen artwork from the playable race. If you notice, they're still using daggers, swords, and guns despite being wolfmen. Admittedly Genn is a weird example, where they show him going beastial for dramatic purposes.