Because coming true from from seed isn't the standard for defining a species or even a variety. For example, Penstemon angustifolius plants normally produce blue flowers, but you can sometimes find white flowered individuals. In this species, white flowers are a recessive trait so if you grow white flowered plants in your garden and they are either self fertilized or fertilized by another white flowered individual, 100% of the seedlings will produce white flowers, i.e. they "come true" from seed. That doesn't make them a separate species or even variety. It's just part of the natural variation within the species.
