Psychological is explained by the fact that individuals

Psychological vulnerability leads
people to procrastinate. Procrastination is a psychological state that makes
people desist from carrying out tasks or making decisions by systematically
postponing deadlines. To make matters worse, the greater the task or the
decision, the more they procrastinate. When they have no alternative left but
to tackle the problem, it is too late to solve it adequately.  Although people realize that postponing
retirement-related decisions carries future costs (to pension income), the
immediate cost to be incurred in terms of time and effort encourages
individuals to avoid the planning.  The
temporal distance between retirement and more immediate tasks lead people to
what is known as temporal discounting (Raaij). This refers to people’s tendency
to prefer immediate rewards to rewards more distant in time. Temporal
discounting is explained by the fact that individuals attribute more value to a
reward obtained immediately than to a greater reward obtained later. Current
pleasures prevail over future benefits. For example, a consumer will usually
prefer $500 now to $520 in a month’s time. Waiting a month for $20 more is not
perceived as a sufficient trade-off. The satisfaction derived from the
immediate reward is overweighed. 
People’s preference for immediate rewards declines and eventually
reverses as the time horizon lengthens. The further a reward lies in the
future, the less value is attributed to it. If the person is offered $500 in
six months or $520 in seven months, they will choose the second option.  People would rather have some money to spend
now (in restaurants, the movies, long-weekend holidays, etc.) than postpone
consumption in favor of a better pension income (a reward obtained in twenty-
or thirty-year time). The tendency to prefer a modest reward now rather than anticipating
a more significant return in the future can be disastrous for saving toward
retirement.