Because the "star" represents the distance and the unknown, while the "sea" represents the grandeur and infinity.
The original source is 1987, the famous sentence of the galactic emperor Reinhardt von Luo Yan Cramer in the novel Legend of the Galaxy, which was first published by Japanese pit king writer Yoshiki Tanaka: "My journey is a sea of stars".
A girl's journey is a sea of stars.
It means: what girls should pursue is a bright and beautiful future.
The earliest source of this sentence can be found in the Japanese science fiction "Legend of Heroes in the Galaxy". The hero said in "Battle of the Galaxy": Our journey is the sea of stars. Now this sentence is generally used to express hopes and dreams for the future.
The journey of a girl is a sea of stars, and there is generally a second half sentence: it is not a world of smoke and dust, which means "not trapped by daily necessities." There is another expression: A girl's journey is a sea of stars. Don't die of love. It means that girls pursue a better future instead of being trapped by love.