Honestly, I don't think they're really birthdays in a way of 'that's when they came to be' anyway, because then you could find faults for all of these dates. Take Switzerland: The 'founding day' got switched from November to August arbitrarily one year, because it was more convenient for a few people.
It's more just a day to celebrate that they exist and what day is better than the National Day?
At least such is my theory! /o/