Why celebrate Christ's birth instead of His conception?

Since life begins at conception, why do we focus on
celebrating the Birth of Christ, rather than His Conception? After
all, from a Catholic perspective, the Incarnation really occurs
about nine months prior to Christmas. Indeed, pro-life movements
around the world have begun using March 25 as a day celebrating the
life of the unborn, during the Feast of the Annunciation, nine
months before Christmas. And why do we pro-lifers throw birthday
parties, instead of conception-day parties? Why do we speak of
being born again? Why measure our ages from the date of our birth,
instead of conception?