Because they use a Christian system. If they don’t want to use a system created by Christians then create their own system.
Don’t use it then make a point of dissing them by calling it something else.
Every culture has a dating system. Most of them are older than the Gregorian system which only goes back to the 16th Century. The world standardized on the Gregorian system for ease of communication. That's a compliment to us, not a slap in the face.