Why is the longer the focal length of the lens, the smaller the depth of field?

Depth of field refers to the object distance range that can form a clear image on the image plane when shooting a scene with a limited distance.

The shorter the focal length, the shorter the axial distance between objects reflected on the image plane, so the images of these objects will be clearer, the larger the range of clear images, and the greater the depth of field;

Conversely, the longer the focal length, the longer the axial distance of objects with the same distance. One object is clear, others are not so clear, and the clear object has a small range, reflecting a small depth of field.