That's because the traditional economic definition of "rational" is an incomplete model of real world sanity that only works in incomplete models of the real world.
In the real world, you have to deal with the uncertain risk that the offer will be unfulfilled due to external factors, which always increases in time relative to present. You, the entity offering you the choice, or the entire environment both exist in, may not still exist in 60 months.
If the same offer is made 60 months from now, you should make a different choice, because the risk of non-existence between time of choice and time of reward has changed.
If the amount of hyperbolic discounting is genetically determined, then is likely that we are still adjusting to the massive drop in risk of death over the past several centuries.
In the real world, you have to deal with the uncertain risk that the offer will be unfulfilled due to external factors, which always increases in time relative to present. You, the entity offering you the choice, or the entire environment both exist in, may not still exist in 60 months.
If the same offer is made 60 months from now, you should make a different choice, because the risk of non-existence between time of choice and time of reward has changed.
If the amount of hyperbolic discounting is genetically determined, then is likely that we are still adjusting to the massive drop in risk of death over the past several centuries.