Right. So, since all of the rules that refer to weapons being written as if a weapon is an external object you hold and manipulate in your hands, treating unarmed strikes as weapons creates some weird, unintended interactions. The easy solution to these oddities is to just say that unarmed strikes are attacks and not weapons. Unfortunately, by the time WotC realized this, the book had already been published and the only two kinds of attacks that existed in the system were “weapon attacks” and “spell attacks.” Rather than leave these odd interactions in place or re-write all of the language surrounding weapons and attacks, they made the call to treat unarmed strikes as an exception to the general rule that “weapon attacks” must be made with weapons. Is the wording unintuitive? Yes. Does it function as intended? Also yes. I’m sure if they could do the whole thing over again, the opposite of spell attacks would be something like “physical attacks” or “martial attacks” to eliminate the unintuitive wording. But that ship sailed about five years ago.