Because of social insurance and self-control. From a new working paper by Pascaline Dupas and Jonathan Robinson:
“Using data from a field experiment in Kenya, we document that providing individuals with simple informal savings technologies can substantially increase investment in preventative health, reduce vulnerability to health shocks, and help people meet their savings goals. The two main barriers that keep people from saving on their own appear to be transfers to others and ‘unplanned expenditures’ on luxury items. Providing people with a designated safe place to keep money was sufficient to overcome these barriers for the majority of individuals, through a mental accounting effect. Adding an earmarking feature reduced savings for the average individual due to the associated liquidity cost and did not help present-biased people save more. For such individuals, stronger incentives to start and continue making deposits are necessary to overcome self-control problems.”