How is this possible when the computation can be done at compile time?

Compilation is divided into several stages. You can think of it as a preprocessing step and switching to machine language. Usually, preprocessors (#include, #define, etc.) are processed in the preprocessing stage, and in the case of constexpr, it is calculated in the compilation stage, whether it is a constexpr variable or a function, and the value can be obtained in the compilation stage. If you have constexpr before the compiler changes to machine language, you can calculate it yourself, and if you can't calculate it, it's an error, and if you can calculate it, it's a pass.