The manuscripts weren't in Greece. These works were written centuries ago, many of them before the rise of the roman empire. As books were expensive back then, they would often be found in wealthy cities like Rome, Alexandria or Constantinople and often taken as spoils after conquests. The fall of the Western Roman empire additionally meant no one had time or resources to go collecting old dusty manuscripts, but the rising Arabic empires did. Later in the medieval period, notably after the crusades and moor conquest of spain, monks began (re)translating the arabic copies of these works, bringing them back west along with many other novel works, commentaries and inventions.