Why the rulers of early civilizations expected people to pay taxes for projects like building canals and city walls?

For rulers like Justinian of Rome or Chinese Emperors for the Great Wall of China, when they taxed the people, they often focused on the lower class, like slaves, and in Imperial China, they had samurais to collect taxes or shared-crops for their services and not doing so was punishable (punishment varies based on time period). Also they paid their taxes for the good of their country so they could thrive with military and have a flourishing society. If you like my answer please give it a Brainiest medal.