Because a right not to be offended requires a power to suppress offensiveness. The right to life and the right to property would be meaningless if murder and theft were decriminalized. In the same way, a right not to be offended implies that offensiveness must be punished.

The power to suppress murder is a relatively small power, and its limits are easily determined by laws and courts. A power to suppress offensiveness would be broad and vague, capable of suppressing anything anyone might object to. Since taking offense is so subjective, it would be difficult for this power to be administered by a neutral third party. It would be prone to abuses.

A right not to be offended requires an arbitrary power incompatible with freedom.