Because if everyone did that, we would live in anarchy, and not in the good way. There is a lot of value in social peace even for those who are on the brink of society, because those are the first to get shafted. Also even a 72 y.o. can have people depending on them, so you don't know if he "didn't have something to lose" - he might have a wife at home which depends on him because of health issues, or he might have children that need support regardless of age.

Although if i ever find myself in a situation where i whatever i can lose isn't worth much anymore, i will probably find an interesting way to go out. From my current standpoint, that day is pretty far away.