1. why do many historians regard the printing press as the most important invention of the past millennium

Much of history would be lost forever if it wasn't written down. Doing it by hand is tedious and not very consistent. Besides the ink fades off over time with some cases. With the printing press, it's much more clear and consistent to read. Also the printing press allowed for anyone to get access to books. Before it was the elite that could only get access to books. With books printed in mass quantities, it basically democratized knowledge and spread it around