Absolutely. And how many merchants, both online or in person that you have spent bitcoin with, use PGP? For me, the answer is zero.
None, because I don't need to. I trust the merchants I transact with. But, even there was a need, a PGP message is not going to save a junkie. Alright, so Bob signs Alice his invoice, Alice sends him the bitcoin and gets no product in return. Now what? Why should I believe Alice for telling me she never got her product and not Bob yelling in craze that he gave it and that she's a liar?
Minimum trust is required. Probably that's why PGP isn't used in invoices.