Traditionally your birth is viewed as the time you officially ‘entered’ the world. So for that reason the stars in that moment is considered to predict the rest of your life, and it sets the position for all the Houses of Heaven in your chart, so the Ascendant is considered to be when your soul entered the world.
But in truth, it is chosen to be that way simply because it’s more practical. If you start a branch of astrology that takes follows the time of conception rather than the time of birth, or whatever time you think the soul enters the body (Some think it enters 40 days after conception), and it grows a bit, in a few years the lives of those born under that system will follow that system.