Here's why:

* Lack of vascular tissue: Non-vascular plants, like mosses, liverworts, and hornworts, lack the specialized tissues (xylem and phloem) that vascular plants use to transport water and nutrients. This limits their size and means they rely on diffusion for these processes.

* Spores as a solution: Spores are single-celled reproductive units that are dispersed by wind or water. They are lightweight and can travel long distances, allowing the plant to colonize new areas.

* Alternation of generations: Non-vascular plants have a life cycle with two distinct generations:

* Gametophyte: The dominant, haploid generation that produces gametes (sperm and egg) for sexual reproduction.

* Sporophyte: The dependent, diploid generation that produces spores through meiosis.

While spores are the primary method of reproduction, some non-vascular plants can also reproduce asexually through:

* Fragmentation: Parts of the plant breaking off and developing into new individuals.

* Gemmae: Small, multicellular structures that can detach and form new plants.

However, spore production remains the most common and effective method for non-vascular plants to spread and perpetuate themselves.