Because they paid £15k for their house in 1974 and it's gone up to £1.5 million through no special skill of their own. Their life expectancy has also increased

Because they paid £15k for their house in 1974 and it's gone up to £1.5 million through no special skill of their own. Their life expectancy has also increased 10 years in that time due to NHS advances so they will be living longer and costing much more than before to care for. The calculus as to what they own and what they owe has changed dramatically just during their life span.