Yesterday was my birthday. I got greetings from a number of friends from here in Atlanta and also from people who had never met me from around the world since my birthday announcement was carried on a couple of the central Bear pages on the internet.
There is clearly no prohibition against birthdays in the Bible. The reason the Watchtower prohibits birthday celebrations is the same reason they discourage college, extracurricular activities in school, etc. They don't want people to be recognized for individuality.
It is a central theme that runs through all of their doctrine and policy.
Sublimate the individual into the group. Make the individual dependent on the group. Hold possible expulsion from the group over the individual's head their entire life.
Diminishing the individual keeps the focus on the value of the group.