Because time began with the universe.
On that note I heard an interesting take on the age of the universe/earth thing -
If one believes that Jesus could do miracles, then when He changed water to wine, how old was the wine at that instant? If wine experts then or now were to examine the wine, wouldn’t they all agree that it was more then 5 minutes old?
So if God wanted something to look like it was 6000 or 6 zillion years old, I believe He could do it.
Why? Your guess is as good as mine.