I wondered the same - symmetry seems quite an arbitrary criterion to me, especially because, for example, humans are not really symmetric once you look at the p

I wondered the same - symmetry seems quite an arbitrary criterion to me, especially because, for example, humans are not really symmetric once you look at the placement of organs. But I could imagine that the mechanism that causes bodies to grow (almost) symmetrically has some deep and unique genetic and biochemical reasons and therefore justifies this classifications.