>why then do modern Christians, including evangelicals, dismiss other cultures so aggressively?

The vast majority of modern Christians doesn't, the influences of Greek culture are readily apparent in the conceptual language of the New Testament, John most obviously when he turns Christ into the Logos. Culturally many pre-Christian practices have been incorporated into for example, Latin American Catholicism. You can literally see it in the architecture of churches.

American Evangelical Christianity is a bit of a different beast and best viewed as a nationalist program that brings particular American tendencies to bear on the religion rather than the other way around.