the arcanists are wearing what they are because the physical model you see is a full NPC placeholder which are often created as one file instead of seperates like gear, its just implimented on top of x gear and y stats because the class isnt really implimented yet.
id bet arcanists will get their own basic armour look too much like CONs and THMs, i just doubt they came up with much so far so theyre just using what we see.