Why people don't do them? and why people do?
Does it makes financial sense to do them?
Pre-nuptial agreements are typically used to protect assets that you have before a marriage in case of divorce. If you don't have a lot of assets to protect, then there's not much point. If you do have a lot of assets, then it might be worth having such an agreement, especially in the case of a second marriage where both spouses have kids from before, and they want to be very clear about what the new spouse gets vs. what the kids get.
Relationship-wise, bringing up the topic of a pre-nup can be touchy, because the other person can feel like you don't trust them, or that you don't expect the marriage to last. And, it is possible that, if you spend too much time and energy thinking about what should happen when the marriage ends, you might not be putting enough time and energy into thinking about how to make the marriage last.