I think because the common usage of birthdates doesn't take the timezone into account. e.g. if you were born on January 1st 9pm PST, that's January 2nd 12am EST

I think because the common usage of birthdates doesn't take the timezone into account. e.g. if you were born on January 1st 9pm PST, that's January 2nd 12am EST – but your birthday is still considered January 1st no matter where you are in the world. So trying to correct such dates for timezones will show users their incorrect birthdate.