I don't get this line of thinking either. If they know who's depicted in the sketch then why waste their own time and resources by asking the public to ID him? That's just hundreds, if not thousands, more tips that they'd have to go through when those man hours could be going to something more useful-like pressuring his peers, continuing the investigation into him, etc. Releasing a new sketch just encourages the very behavior that LE has tried to thwart-like comparing pictures of real people to him on social media. It's opened up a whole new can of worms that they're now probably trying to slog through.
Brookelyn Farthing is an 18 yo who disappeared close to me. In fact, our farm has been searched more than once because it's nearby and we have a lot of acreage. In that case, LE and the rest of the county knows the "who" but not the "why" or "where." It's been incredibly frustrating not to have an arrest, but we also know that for the past few years LE has been searching for evidence to tie the guy(s) to the crime. They're interviewing and pressuring all the people who may not be directly involved but possibly know something. Searches are still being undertaken. Stuff is happening in the investigation to move it forward. They're not asking the public for identification.