I have come to understand advertising a lot better since I asked, while looking at some lingerie ad, "Who would ever want to wear something like that?" and someone replied "This ad is for your spouse who wants to see you in this."
The same reason that underpants come packaged with pictures of hunky guys on the front. The guys aren't there for the men wearing the underpants, so presumably they're there for the women buying them.
They don't crop the faces off the model's picture only for costs. They don't have a face so that it's easier to project yourself with said body.
Meanwhile, I research hiking trips almost every day of the year, talk online about hiking frequently, and I can't remember seeing a single ad for hiking gear or anything remotely related.