I-795's intended purpose is to serve as a shortcut to Wilmington, so why not sign it as such?
Not when it was originally signed. The original US-117 relocation had two original purposes:
1. Providing a connection to the north for the Global TransPark in Kinston (which is not along I-795).
2. Providing a connection to the north for container traffic from the Port of North Carolina in Morehead City (which is also not along I-795).
Since all of it was [kinda] constructed to Interstate standards, NCDOT applied for the Interstate designation the next year. It probably surprised NCDOT when AASHTO approved the designation. I'm sure that it wasn't long after receiving the I-795 designation that the locals in Dudley started noticing an uptick in through truck traffic on US-117 between I-40 and Goldsboro. So yes, I-795 now is planned to be extended to I-40 and should get eventually get "Wilmington" as a control city. But the current BGS on I-95 and the BGS pull-throughs on I-795 are not large enough to accommodate both "Goldsboro" and "Wilmington" on the same signs. In proper North Carolina fashion, everyone will need to wait until the old signs are ready to be replaced (or I-795 is extended, whichever comes first).