It’s not. It’s the same price as the iPhone 6 when it was introduced.
What people forget is that they have a pretty powerful computer in their hands, not just a cell phone or texting device. An iPhone does a lot of the things a laptop or desktop computer does, yet it fits in your pocket. Usually, when you buy something that is smaller, it costs more (a desktop computer vs a laptop, for instance).
Also, an iPhone 6s is comparably priced to the equivalent Samsung Galaxy models, so I really can’t agree that it is “so expensive.”