Why don't we envy future persons? The question is Chris's. It's posed by him in the context of noting that generally we aren't troubled by the possibility that future persons might be better off than we are in various ways. I have two suggestions.
One reason is uncertainty. We can't know the overall situation of the putative future persons whom we're considering whether to envy or not. They may, for example, be a bit better off in some respects, but much worse off in others, so that our envy would be 'wasted'. For all we know, their situation may be dire. Envy doesn't work that way: fixing on people distant from us in time and whose state of being we can only speculate about; and then feeling envious of them on the chance they may be better off than us.
My second suggestion follows closely (in a logical sense) on the first. Envy isn't merely the abstract thought that, were I given the choice between p, q, r, s etc now (my own current bundle of circumstances, goods and not-so-goods) and t, u, v, w, etc in the year 2060 (someone else's bundle), I'd rank theirs above mine. Envy engages the emotions; indeed it is an emotion. It tends to fasten on others relatively close to, rather than distant from, the envious subject. The context of Chris's question calls for the envied to be so distant that they don't even exist yet. Envying them isn't merely futile, it's pyschologically hard.
And that's if you're given to envying anybody in the first place.