simply put, it's the most prominent "weird" fetish out there that's also the least offensive to people who don't have it. Most people who don't have the fetish will either decide it's doing nothing for them and move on to the next image or keep watching the video to get to the parts they do like, while its inclusion will draw in people who do have that fetish. So, for an artist, it's not really a risk to do it, like another fetish would be. Like, say, a pee fetish scene, while people with that fetish may appreciate it, others who don't like it may not only leave the video, but may skip out on future videos.
Personally I enjoy the scenes, but not as much as armpit based scenes (yes, that's a thing)