Because the language defines it to be an atomic operator named "conditional operator." As to the implications of that definition, please read the referenced link[0].
> In fact, I'd argue that the only reason this might seem seem like a rule is that almost all of the other operators in common use are either unary or binary, making it easy to use a single token for the operator itself.
It is "a rule" because that is how the language is defined. If you'd like to argue otherwise, please feel free to do so with the C++ standards committee.
0 - https://en.cppreference.com/w/cpp/language/operator_other
