NOT a reason why a high dependency rate leads to low levels of social welfare (from a list in a common question of this type): Resources are disproportionately spent on the oldest members of society.
Spending resources on the oldest members of society would, in fact, be a demonstration of social welfare in action. It is also the case that those in dependency are not just the oldest members of society, but especially also children. If there are high percentages of children compared to those of wage-earning age, the amount of resources available to spend on social welfare programs will be affected by that.