How does not filling a cavity lead to tooth loss?

Progressing decay: It may take some time (months-years)but cavities are caused by bacteria which don't die easily. They reproduce and continue to cause probs, they will eventually get to the nerve, to the bone and destroy the structure of the tooth leaving it unrestorable.

Fracture of tooth: By not filling a cavity, the cavity will get larger and the tooth becomes weaker. At some point, the cavity will reach the nerve which will cause more pain and possibly an infection, or the cavity will get large enough that the tooth fractures. If the fracture occurs along the root of the tooth then the tooth may need to be removed.

The cavity will: Keep getting larger and larger. Eventually, the remaining tooth structure will not be large or strong enough to support a restoration (filling) and thereby determined to be non-restorable. A non-restorable tooth needs to be extracted before an infection/pain develop.