The reality: It depends on the state, the assets, and the family situation.
In community property states (Arizona, California, Idaho, Louisiana, Nevada, New Mexico, Texas, Washington, Wisconsin), spouses generally own half of all assets acquired during the marriage. But even in these states, separate property, assets owned before marriage or received as gifts or inheritance, does not automatically go to the surviving spouse.
In common law states, the surviving spouse's share depends on whether there is a will, whether there are children from a previous relationship, and the specific state's intestacy laws. In some states, a surviving spouse may receive as little as one-third of the estate if there are children.
If you are in a same-sex marriage, a domestic partnership, or a long-term relationship that is not legally recognized as marriage, the automatic protections are even weaker. A will is essential.