People usually have retirement plans. Young people have ideas and expenses, but not resources. Old people, conversely, usually have resources but don't really know what to spend them on, whom to give these to.
So they give the resources to banks, which in turn themselves are for-profit entities, and this creates a positive feedback loop.
Interestingly, the old people, I believe, don't really need maximum profit… but their money are invested exclusively in such things…