Because 9 months is vaguely close to an average date from conception so you can't just backdate it from birth and have it mean anything. And many people don't know the precise date of conception.

Meanwhile, it's pretty unambiguous when you deliver a baby.

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“How old you are at birth” varies depending on the culture. If you follow eg the long standing Korean method, I’m two years older than I am if you decide that age at birth is 0 months old. (South Korea is changing to the 0-at-birth scheme soon.)

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We as a society have decided that the time you spend inside your mother does not count as your age, and that's why we celebrate Birth Days to mark the passing of each year.
We could get more medical or scientific and say that while you're inside the uterus, you're still developing as a human being and you're not formed 100%, so we decide that when you get out, it's because you're fully formed and ready to rock the world.
In Korea if i recall correctly they are 1 year older because they account for this time, but again, it's a decision as a society.

Finding out the exact date of conception is basically impossible and historically the date of birth or baptism was the basis for age in western countries. Expecting everyone to change the convention for something like this is just unrealistic.

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If you believe in the Bible then you know that the soul enters the body with the first breath. A fetus does not have a soul, so the time in the womb doesn't count.

Besides, 'birthday' literally means the day you were born.

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Because humans count BIRTHdays. Plus, conception is harder to pinpoint, exactly. Therefore, counting in years, from the date of birth, is the standard worldwide. (Except in societies or social groups or religious groups where birthdays are not celebrated.) This has been the way, going back as far as 3000 BC, at least, when there is documentation of an Egyptian Pharaoh's date of birth.