Media tend to gravitate toward simplicity. Saying "assault weapon" is simple, just as the terms "mass shooting" and "common-sense gun control" have become go-to terms. Stylebook or no, so long as popular media almost exclusively depict ARs and AKs in war or in criminal shootings, spraying bullets by the hundreds, people will continue to be conditioned to associate those guns with "assaults" and thus they remain "assault weapons". Also note this is only the stylebook of the Associated Press, not of any other medium that chooses to go a different way.
I also tend to agree that this may not be a good thing. Right now I suspect many non-gun people distinguish what they think of as an "assault weapon" from other rifles and pistols. What will result from continuing, unending reference to a bad person's use of a "semi automatic"?. Will they begin to associate any semi-automatic with "bad"? .