Sure. But going in, and probably throughout most of their marriages, they were just as confident in the health and general wonderfulness of their relationships as you are in yours. People don't realize that they're deluding themselves until suddenly they aren't.
What you guys don't realize is that a prenup isn't about saying "A gets everything and B gets only what s/he brought into the marriage." It's about deciding, upfront, what would be equitable in a variety of situations, and that process really shows whether people are on the same page - for instance, if A is going to be the homemaker, whether A and B value that role equally.
If you can't come to an agreement about such things upfront, then really your relationship isn't very strong or very honest.
I can't agree with this more. Why don't more people discuss all the important issues up front cause I've been amazed over the years the number that don't.