Why Should We Discount Future Lives?

Homo sapiens has been around for about 300,000 years. That would mean there's 700,000 years still to go - and we're not a typical mammal species. If you were to keep discounting well-being at the standard kind of economist's rate of 2% per year, that would mean actually you should save the one life in 10,000 years time. And I just think that's absurd. At least past some point into the future, just harm is harm, benefits and harms, they count the same, no matter when in time they occur.