People work not just on their own behalf, but also of their descendants. Knowing that your spouse or kids will keep getting royalties provides an extra incentive to produce new work, much like people create businesses or make investments to leave as inheritance.
how often does it profit large corporations and not their descendants?
Rarely; it usually profits either both or neither.
It doesn't; it's just there because Disney has enough power to make it so.
It incentivizes artists to create art, because their work will support their heirs.