Why isn't Noah a saint?
Formal canonization is a process that developed hundreds of years after the founding of the Church. Before that, righteous people were "recognized" or "acclaimed" by the Christian communities to be saints. That is why Old and New Testament saints (including the Blessed Virgin Mary) were never formally canonized. Noah is one of the righteous people of the Old Testament who are generally considered to be in heaven. Indeed, in the "hall of fame" chapter of Hebrews, which records the righteous deeds of Old Testament saints, Noah is included in the roster:
By faith Noah, being warned by God concerning events as yet unseen, took heed and constructed an ark for the saving of his household; by this he condemned the world and became an heir of the righteousness which comes by faith (Heb. 11:7).