31st August more important than 16 September?

I began to wonder, why is this Isn’t 16 september the actual date when Malaysia is born? 31st August is just the independence of peninsula, known as Persekutuan Tanah Melayu 1957. How about Sabah and Sarawak? We have not yet archieve our independence that time. We’re still under the British until the late Tunku Abdul Rahman suggested Malaysia. And Malaysia is born on 16 September 1963.

p/s: Everyone seems to forget that 16 September. I think this date is a much more important date than 31 August.