Because a thread with only replies like "hey" or "welcome" doesn't contribute much to the forum and seems kind of pointless to me.

The birthday thread is there because we wanted to decrease the amount of birthday threads cluttering the topics list (it's kind of a tradition here, congratulating people with their birthdays) and a central thread seemed like a good way to do that. We seriously thought about forbidding birthday threads overall, but in the end decided to go for the thread.