Because the ficticious literary character Hanibal Lector uses one. Now before you go all 'that's nuts' on me, have you ever been to SpyderCo's Corporate Headquarters in Golden, Colorado? Go in the showroom and ask to see it. There's 2, a Harpy & a Civilian. SpyderCo is trademarked and they were paid for the use of that knife name brand. It's called Marketing and probably the biggest reason anybody 'gets' anything about odd shaped knives.
Or, because people like the ergonomics/steels/cutting geometry. Same as any other brand. Hardly anyone knows that it was a Harpy in Hannibal, and Spyderco was selling fine before that. Your logic doesn't pan out, but ok.
I'm not into the Contego(too weird a handle, not ergonomic at all), the Adamas(too wide a handle, better off with fixed), most of the larger ZT knives, and any other knives that can't cut worth a flip because they're too thick to part material easily. Sure, you can sharpen them to a crazy edge geometry, but you can't take away the width of the rest of the knife without extensive work.
Any knife that uses a typical, common lock, as in framelocks or lockbacks, and yet is ridiculously overpriced because of steel, maker, or fit&finish. That includes CRK, a lot of ZT, some Spyderco, Strider, Hinderer, Medford, Direware, Grayman, etc etc. I understand that it is easier for them to use a framelocks as it is both free to borrow, and fits simply into their designs. Still an outdated lock(and sooo many people gripe about framelock issues on this forum).
Also, any knife that goes beyond "built for use and also looks good" into the realm of "this is three times the price because we make it 3% better". I use my knives, I don't collect them, and I am not rich enough to throw away money. I can afford Sebenzas, Striders and such, but I always ask myself why I would need a cutting tool that costs as much as a car payment, cheap washer or dryer, some guns, a cell phone, a tv, a nice complete camping setup...I'm just randomly listing, but basically there is always something that is more deserving of that money. I think the only two I'd make a concession for would be Andrew Demko and William Henry.