Solid, gas, and liquid are taught as the states of matter simply because those are the only states of matter that you’re ever likely to experience. Plasma is sometimes included because it’s easy to demonstrate (e.g. neon lights) and it fits easily into the same continuum as solid -> liquid -> gas.
There are *dozens* more states of matter, but they’re either so esoteric that they’re difficult to explain in a basic science class, or so exotic that most people will never encounter them, or both.
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