One argument for why people don’t proportionally care about future generations is because they’re such a distant concern. A pattern I notice with the moral shifts you describe is most people have become more distant from the relevant populations over time, such as prisoners and animals. We’re also more “distant” from our ancestors and deities in the sense we may care about them much less in large part because we’re exposed to memes promoting caring about them in our everyday lives much less frequently.
One argument for why people don’t proportionally care about future generations is because they’re such a distant concern. A pattern I notice with the moral shifts you describe is most people have become more distant from the relevant populations over time, such as prisoners and animals. We’re also more “distant” from our ancestors and deities in the sense we may care about them much less in large part because we’re exposed to memes promoting caring about them in our everyday lives much less frequently.