Because Christians are generally speaking quite ignorant of the Old Testament, and not infrequently the New as well. Every religious Jew on earth is well aware of this, and if you read the two versions side-by-side you'll see the differences are stylistic and not substantive--the shade of difference between "honor" and "love" or "remember" and "observe" is just not that great, though our practices often are explained as owing to these distinctions.

If you are looking for something about the Bible to be upset about, you can certainly do a lot better than this.