You can make things out of plastic that has short carbon fibers in it, but that is not at all what is being done here. Carbon fiber (as done in a lot of things, including bass necks) is a composite material - long carbon fibers (which can run the entire length of the neck, and can be oriented however the designer wishes), which supply the tensile strength and stiff ness, and a glue - epoxy or polyester resin, which binds the fibers together. It's a VERY different material than any simple plastic, and has far different properties.
Wood is also a composite material - it has fibers (cellulose, which can run the length of the neck, but which run the direction that they grew in the tree) and it has lignin - a natural glue, which binds those fibers together. Wood is lighter than carbon fiber - maple is about a third the density of a typical carbon fiber layup - for a carbon fiber neck to be a decent weight, it has to be hollow.
Both wood and carbon fiber are anisotropic materials - they have far different stiffness and strength along the direction of the grain than they across the grain. You can make a neck that will resist the forces of strings tied to them with wood or with carbon fiber. If you try that will plastic in the normal cross sections you would use with wood or carbon fiber, it won't work at all - it won't be stiff or strong enough. It will also be heavier than maple - plastic is about 67% more dense than maple.
