Because Daddy was born on that day.
Birthdays - and the annual observance of these times both as a society and for individuals - is the oldest of celebratory rites, and one that arose in every culture as soon as they had any sort of calendar.
The first real calendar has to do with the sun and the moon's position - the fundamental tools of astrologers today. The wise men (and women) noticed that when the sun hit the same spot in the heavens (against the zodiac backdrop) that it held on a person's birthday ... well, that day turned out to be extremely fortunate.
Maybe a woman found a husband, a shepherd saved his sheep, a merchant sold his goods or an illness vanished. Plus, people gave to the birthday person instinctively. So presents were bestowed - both from others and from the universe. The wise elders saw this phenomenon happening all the time. And just as most basic astrological information came about based on these days, the astrologer/priests saw that a "pattern" was occurring.
This lucky pattern brought joy, and thus the birthday person wanted to celebrate. His or her friends naturally gave to them (thus the idea of presents) and the birthday celebrant wanted to share his or her pleasure, so a big gathering was held.
That's the reason that we celebrate birthdays today - as a tradition that came about because of repeated observations of luck on this day.