Why discs instead of cartridges, which are currently the superior physical option? I personally try to buy physical whenever possible, because I don't trust companies to not ban my account and flush hundreds of dollars of games down the toilet, and it generally feels better to have just that little extra bit more ownership over my own property.
Because cartridges cost more to produce and are limited in storage. Switch carts cap out at 64gb, Blu Rays are up to 100gb at this point and it's much cheaper to chuck a few of them in a box if the game goes over that. Hence all the switch games with massive downloads required on top of the cart.