Because 'generate a table by having make run a simple program' is something people have done for decades with C, C++ and other languages, whereas constexpr is a

Because 'generate a table by having make run a simple program' is something people have done for decades with C, C++ and other languages, whereas constexpr is a new feature that's only just appeared in the C++ language spec and apparently silently degrades to "not actually at compile time".