If the bean seeds are examined closely, it is apparent that they contain only a embryo and no endosperm. why is that ?

Bean seed is a non-endospermic seed. In this type of seed the embryonic leaves or cotyledons occupies most of the space within the seed. The embryonic leaves contains food materials which provides nutrition to the growing embryo. Therefore, non-endospermic seed does not have endosperm. Endosperm in endospermic seed provide nutrition to growing embryo, example corn seed.  

they float because they are less dense than the water itself

something denser than water, like a rock, will sink to the bottom