Because that's what we are. In the South, you are raised never to call ANYONE by their first name if they are older than you. Unless you add the repectful "Miss" or "Mr" in front of it and only if the older person requests it. For instance, I would be Miss Peaches. "Miss" applies if the woman is married or not if you use their first name.
If you don't want YOUR kids to call you Mom or Dad, that's up to you. My son used to call me by my first name when we were at events that had a lot of mothers there - if he said "Mom!" all the mothers turned to him ;)
The transformation from Mommy to Mom is always a tough one
I remember being at a dinner with my son and my mother. One of his friends came to talk to us and when they left said "Bye Ms. Smith". My mother said "Bye!". Then realized they were referring to me - it never dawned on her *I'D* be a Ms. Smith some day, lol!