ply comes from technology of diagonal tires that used fabric for reinforcement.
10 layers of fabric is stronger than 6 layers.
But for last 50 years cars are using radial tires with steel mesh so no more multi-layers of fabric.
Still 50 years is not long enough to wear old perceptions, that's why it is still listed this way.
Kind like cellphone manufacturers with technology change were still putting dummy antennas on the phones, since it took several years for owners to trust the phone with no antenna.
Ply-rating or load-rating this is just nominal category. In each category the tires will have individual weight listing and that is what I am checking - choosing one.