Birthdays are nothing if not emotionally loaded. The pressure to celebrate the anniversary of your existence once a year is wrought with worries about who will show up, whether you'll have a good times, and the passage of time.
Of course, when you're still a child the existential worries about age and time are less prominent than the desire to get lots of presents from adoring classmates! I'm not a parent, but as an aunt I've witnessed firsthand the anxiety around organizing a party for a child when you're not sure how many kids will show up.
Birthdays are hard enough to navigate as an adult, so when you add 6-year-old emotions it's a whole different ballpark.