[sltkr]

Because `constexpr` was not supported in C until very recently (C23).

And `static const` by itself is not a good alternative, because constants can't be used in a lot of places, for example when declaring the size of an array.

[nj5rq]

Constants can be used for array sizes since a lot of C standards ago, right?

[PhilipRoman]

They end up as a VLA which may or may not be what you want.