Free: Contests & Raffles.
Your question asks why they exist, but you only discussed your hunting needs. I'd venture to say they're used a lot for target shooting still. Shoot a rimfire with a fixed 12X and you'll be hooked. The ACOG on my M4 in the military is also a fixed 4X. I think ACOG makes them from fixed 1.5 to a fixed 6X with Trijicon sights. Having said all that, I don't think I'd want a fixed power scope on a hunting rifle generally, but for quick shots the 4X still works for me, at least in a military application.