Asking why a fetish isn't common is kinda oxymoronic, in a way, considering all true fetishes are pretty rare. If you just mean 'why aren't they seen as attractive by straight women/other male-attracted groups' then I can assure you they are.

The whole neck, collar bone, shoulder and peck area is like, my favorite part of the male form, and I am not alone by a long shot. Maybe talk to more women about this stuff lol.

Lots of us appreciate several parts of male anatomy in similar ways that straight men do female anatomy.