True, up to the point when the designs are finalized and tooling is complete. At this stage, production volume make the profit margin on variables better, and manufacturing fixed scopes is not as profitable anymore. The whole process is driven by creating the "need" (want?) for variables, be it with rifle scopes or other optics. At the same time, as Dogleg notes, he finds the variable power scopes useful in the environment he hunts and the way he is making use of the reticles - a clear, concise, and logical explanation why this works for him. Perfect!
Returning to optical qualities in general, a fixed SLR camera lens beats a zoom lens, any day. A zoom lens may have a sweet spot where the differences are negligible, but generally the extremes of focal length are compromises. But these days, fixed focal length lenses are rare, except for specialty work (eg. Micro Nikkor, super wide-angle lenses, etc., and these command a premium price - laws of demand and supply in action.) As to the OPs question re: spotting scopes, I venture to guess that the same rule applies re: image not being equally sharp at the extremes, especially at the high magnification.