why do people say xmas instead of christmas

The abbreviation "Xmas" has been used for centuries as a way to shorten the word "Christmas." It's derived from the Greek word "χριστός" (khristós), which means "anointed" and is often translated as "Christ" in English. So, by using "Xmas," people are essentially referring to "Christ's mass" or "Christ's feast," which is the celebration of the birth of Jesus Christ.