What ... as opposed to the "later, fake Church?" They didn't celebrate because, culturally, 1st-century Palestinians didn't celebrate the birthdays of commoners.
Why are Pagans automatically "sinners?" Origen said "sinners," not "pagans." And Origen wasn't Catholic. According to tradition, he was Egyptian, and spent a lot of time working in Alexandria. In other words, he was more Eastern than Western.
There's no command to celebrate Easter, Thanksgiving, All Saints' Day, Pentecost, or the Transfiguration, either. So what? The Bible is not the entire compass of Xy. Celebrating the Incarnation is a Christian tradition. True, it was begun later than Christ. But arguing that "this isn't something Jesus would have done" is a red herring, in this case.