Why isn't Noah called a saint?
The short answer
Formal canonization developed centuries after the Church began, so Old and New Testament saints — even the Virgin Mary — were never formally canonized. Noah is among the Old Testament righteous generally considered to be in heaven, and Hebrews 11 lists him among the heroes of faith.
Canonization came later
Formal canonization developed hundreds of years after the founding of the Church. Before that, the righteous were recognized or acclaimed as saints by Christian communities. That is why Old and New Testament saints, including the Blessed Virgin Mary, were never formally canonized.
Noah's place
Noah is one of the righteous of the Old Testament generally considered to be in heaven. The "hall of fame" chapter of Hebrews names him: by faith, warned of what was not yet seen, he built an ark to save his household, condemned the world and inherited the righteousness that comes through faith (Hebrews 11:7).