They do something else to earn a living, and do landscape photography on the side until they either give up trying to do it professionally, or they find that there's enough demand for their images that they can let their landscape photography pay their way.

Ansel Adams taught piano for many years. Even after he became a photographer full-time, most of his income was from paying commissions (where he did what a customer asked him to do, usually not landscape-related) until well into his life. Getting paid enough to be a landscape photographer to make it your exclusive and sufficient source of income is not easy.