Dual conversion "online" ups vs other power device

Having read about the benefits of power conditioning, I have to ask why dual conversion UPS are not used more often? If one was to get a sufficiently powered model, say 3000watts upwards, this should be enough to satisfy all but the most power hungry systems out there. Surely generating "clean" power would be better than any device trying to work with the existing incoming power and trying to modify it. Am I missing something?.