Because the modern nation of Australia has a national character borne in its history as a British colony, and the arrival of the first fleet is a day of high significance in that context. That’s it. People who heavily shaped the future of the country arrived and proclaimed ownership of the land for England. Are you seriously arguing that arrival of the first fleet is not significant? Or that it’s not appropriate?You mean, too factually accurate for you to contend with.
But hey, draw on your depth of knowledge of Australian history and educate us all - why is 26 January 'an appropriate date' to celebrate 'the birth of a nation'...your words??