When people think about a will that is not valid, they often think about a fake will. Maybe, after a parent's passing, an heir drafted a will that gave them a far greater share of the estate than they got in the original. There is a clear chance for financial gain.
You have $10,000 in credit card debt. If you pass away, does that debt simply go to your children? Are they obligated to pay?
One reason people sometimes give for not doing their estate planning is that they don't think their estate is complex enough. Maybe they don't have multiple homes or properties. They don't have complex investment portfolios. They don't want to use trusts or give to charity. They just assume that the assets they have should go to their heirs, and that's fairly straightforward.