I think there's a point in rjeli's and skrebbel's comments though. This wasn't designed in from the get go; it started as a hack that got papered over with abstractions and accepted as a standard practice. Despite later improvements, the whole thing still has the aftertaste of a clever hack.

Using a modern example, the way template metaprogramming started feels like return-oriented programming, or those "MOV is Turing-complete" or "MPU error handling is Turing-complete tricks". Someone figured out the standard unintentionally lets you make a conforming compiler do computations, and it ended up becoming an accepted feature, instead of updating the language to something resembling modern constexprs, or Common Lisp's compiler macros.