It seems that this is not because little boys like to play with mud, but because children like to play with mud.

Due to the amorphous nature of mud, it can be made into different shapes and produce different "works" by squeezing and kneading. This can not only satisfy the exercise of children's fine finger movements, but also cultivate their creativity. Children's imaginary things can be shaped by themselves, which will make them feel a sense of accomplishment. So I think this should be a little reason.