Because that is what people are willing to pay for them.

A lot of those types of goods are priced extremely high compared to the actual cost to manufacture. Lighting is another example of this. You are paying for the design and brand in many cases even though they aren't built better or with better materials compared to a more moderately priced faucet. High end faucets do have some more expensive components in them than cheaper ones. For example, orings are replaced by ceramic cartridges that feel better. Those changes maybe account for a couple dollars of manufacturing cost though.

It can be frustrating knowing how much stuff costs to manufacture, so often I look at items like this and get frustrated because I know the manufacturer is selling for >10x what it cost to make.