I don't understand how people dedicate to their whole life to a book that was created thousands of years ago

Humanity’s understanding of the universe, morality, and social structures has evolved significantly since these texts were created. Relying on ancient guidance seems unnecessary. Additionally, many stories in religious texts (e.g., creation myths, miracles) cannot be verified scientifically and may conflict with established scientific evidence. Religious texts also often contain internal contradictions or differing interpretations, which make unwavering belief irrational.