Omnisphere has a huge arsenal of samples that are unique, if not unlike anything available anywhere else.
Bowed bicycle rims, a bunch of
Diego Stoccos custom/modified instruments, various instruments played with everything from dental floss to electric toothbrushes,
Lightbulbs, & pianos burning of fire.. Not to mention a ton of of completely normal organic instruments, analog and modular synths all sampled immaculately.
While there's plenty of stuff Omnisphere has that you could find a similar replacement for in Kontakt, there's also tons of stuff in there that no Kontakt library comes close to touching, even 12+ years after Omnipshere was released....
Anyone who owns Omnisphere and doesn't fiddle with the included content to make their own patches might as well be getting half the value of the cost of Omnisphere they spent when they bought it. It literally can cover any genre of music imaginable. And there's plenty there to cover SFX design as well...
If you only use presets I could see your point. But if you aren't afraid to design patches yourself, even using basic settings, Omnisphere has so much more to offer than you can wrap your head around until you actually own it. As a rompler? An expensive choice... (Even then it's not like it's pricy compared to what $500 in Kontakt tends to get you...) As a sound design instrument it's still the best spent money I've spent on any virtual instrument by a mile.... 7 out of 10 times it's the 1st thing I reach for.