Because the smallest apertures are a tradeoff, not a free gain. Stopping down does increase depth of field, but very small apertures like f/22 or f/32 also reduce image sharpness due to diffraction. In practice, many landscape photographers choose a middle aperture such as f/8, f/11, or f/16 because it often gives the best balance of depth of field and overall sharpness.

They also use focusing technique to maximize what looks sharp. A common method is focusing near the hyperfocal distance, which can make everything from a certain near point out to infinity appear acceptably sharp without needing the tiniest aperture.

There are other practical reasons too: a smaller aperture requires a slower shutter speed, which can be a problem if wind is moving trees, grass, or other elements. And sometimes photographers intentionally use less depth of field to emphasize one part of the scene.

So the goal in landscape photography is usually not “use the smallest aperture possible,” but “use the aperture and focus point that give the best overall result.”