Because people generally cannot work anymore at a certain age, and most western countries do not think it is acceptable to let elderly people starve, even if they could have taken decisions to prevent that.

Because you cannot expect all of the population to individually make wise financial decisions spanning multiple decades well past the years they can even generate income if they wanted. A lot of people already have problems budgeting for next month.

Because spreading out the "risk" of living much longer than you anticipated prevents destitute 95 year olds.