Here's why:

* Non-vascular means that moss lacks the specialized vascular tissues (xylem and phloem) that transport water and nutrients in most plants. This limits their size and means they must absorb water and nutrients directly from their environment.

* Plants are multicellular organisms that produce their own food through photosynthesis.

Moss is a member of the Bryophyte group, which also includes liverworts and hornworts. These are among the oldest and simplest land plants, evolving before vascular plants.