Does anything justify it? Maybe. But increased complexity requires it, and consumer demand allows it.
It is obviously not true that all zooms are more expensive than all primes — those cheap kit zooms can be had for less than any new prime (with only a few 50mm f/1.8 options being in the ballpark). Those lenses are optimized for price, though, not image quality. If you want a zoom lens with better image quality, especially one rivaling that of a prime in the same focal range, you need more complexity. And that means more materials, maybe more exotic materials, more complicated engineering, more points of failure, and so on.
That costs the manufacturers more. The economic choices are: a) make them anyway and eat the cost, b) don't bother making them because they won't sell, or c) pass that cost on to the consumer.
Choice "a" probably actually comes into play at for the kit lenses, where zoom is an important enough consumer feature for entry-level kit buyers that those lenses are priced below the normal markup. And "b" is probably one of the reasons you don't see too many competitors to the Sigma 200-500mm, although size and weight probably also factor in. Mostly, though, it's "c": many, many people buy zoom lenses at current prices.
If you don't find the advantages compelling, lucky you! You can buy prime lenses instead.