Only because there's not universal agreement on what constitutes a species. We make organisms at work all the time that can no longer interbreed with their original species, only among themselves.
Furthermore, evolution assumes that higher life-forms evolve from lower life-forms.
I've never seen a working statement of this anywhere, except from Creationists and ID'ers. I know what you're trying to say (I think), but if you start with biofilms and anything at all happens, you're bound to get something more capable than slime. (I had to work the "slime" angle in.)
The reason there is no agreement for what speciation is, is because the changes are hardly discernible either genetically or in any other way. A small change can be entirely due to a selection of different paths to react to the environment. We know that the body reacts to the environment in many ways - attacking intruding bodies for example, regulating body temperature and many other ways. So making small changes to adapt to the environment need not involve any change in the genes at all but just the turning of different genes on to act to adapt to the environment. Indeed, this is the most likely cause because many of the examples of speciation claimed by evolutionists are way too fast to allow for a mutation to have achieved such changes.