Because your age is your birth age, not how old your body is.
That "birth day" can be measured exactly and celebrated as an event people will remember.
It keeps things simple.
There's little benefit to adding on your womb time as that will vary from person to person and make it more complicated to remember or validate.
Few will be born exactly 9 months after exact conception.
Historically it was simper to measure when you left the womb.
