I have started studying quantum computing with the effort to model "human behavior". In view of this research effort, I am posting a fundamental question.
It doesn't have to be. We can use any other 2-level system we want to make a qubit. For example, with a SQUID you have the states of something akin to a pendulum oscillator. You pick the parameters so the two lowest states are isolated from the others so you effectively are only working in that 2-level system. You can also do other systems as long as you've got $d$ states separate from the rest that you know how to control and then you can make a qudit with $d$ instead of binary.