Why did the builders surround the molding with boards on the outside when they poured it? Couldn't they have used other materials?

It doesn't have to be wood. There are also iron formwork, aluminum formwork, etc.

Because the columns and beams of each building are different, if you use wooden boards, you can be more flexible according to the actual length and shape, and the workers will saw the wood into the right size. With the iron template is too heavy, also need wooden board with use. Aluminum template is too expensive, almost no domestic use.