Why would Parents ignoring their grown children’s birthday?

What is the phycology behind parents ignoring or doing the bare minimum for their child’s birthday? Both of my parents just text “happy birthday” and that’s the furthest it goes. My parents are divorced and I have a family of my own. I talk to my parents often and we are not estranged. But my parents have never made a big deal about my birthday for as long as I can remember. My mom would call me the day after my birthday when I was a child. When other family members have birthdays we will celebrate with family and cake, but no one celebrates me. If someone has a birthday close to thanksgiving or Easter, we will have a cake and sing for them during the celebration. My birthday is close to Christmas but no one sets a candle out for me. Am I being immature or are they neglecting me?