If reason is primarily optimized for arguing about right and wrong, we shouldn’t be surprised if this leads to some characteristic biases when we try (or think we try) to use it for truth-seeking. In particular, we tend to be more successful in argument if the ethical principles we state are more general, less based on personal preference, and more widely agreed upon, and so we’d expect to see a bias in favor of these characteristics. Carried to its extreme, this would lead us to characterize our moral framework as universal and objective, and to discount any moral framework which fails to make those claims.
An addition to the evo-psych account:
If reason is primarily optimized for arguing about right and wrong, we shouldn’t be surprised if this leads to some characteristic biases when we try (or think we try) to use it for truth-seeking. In particular, we tend to be more successful in argument if the ethical principles we state are more general, less based on personal preference, and more widely agreed upon, and so we’d expect to see a bias in favor of these characteristics. Carried to its extreme, this would lead us to characterize our moral framework as universal and objective, and to discount any moral framework which fails to make those claims.