You can build a good boat for perhaps half the price or less of a NEW boat of similar performance. If you are a good scrounger or patient for a deal you can build a good boat for not too much. Some of the new small boat designs, especially some plywood ones, can be made for comparatively little cost and in comparatively little time. And finally there may be special reasons to build a boat, such as a family or youth group project, or to make a gift for someone with extra value because you made it. Boat builders understand that intangibles are important.
Wood is pleasant to work with.
Most or all other materials are unpleasant. Since recreational boat building is properly done for the pleasure of building, wood is a good choice. There are many more good plans for wood craft than for other materials. Traditional designs are in wood. These are proven designs.