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.
Birthdates are particularly difficult to handle properly in a database if there is even a slight chance that you'll need to enter a foreign immigrant into the data.
Just for instance, older Iraqis (if I remember correctly) tend to be part of a culture where their individual birthdates were never recorded, and each person was considered 1 year older on some September date (maybe even talking the Islamic calendar there too). Should you store that date in your database, pretending that it means the same thing as an American who knows they were born at 7:08pm EST on July 17th, 1977?
And that's just one example of many.
I think I learned about this one because intelligence officials were freaking out when they eventually noticed all these people sharing a "birthdate", and it made the news.
Data about human beings is highly un-normalizable, as evidenced by everyone trying to stuff anthroponyms into the American "first name, middle name, last name" paradigm.
