What's the point of telling the compiler specifically to include the file only once? Wouldn't it make sense by default? Is there even any reason to include a single file multiple times? Why not just assume it? Is it to do with specific hardware?
There are multiple related questions here:
#pragma oncenot automatically enforced?
Because there are situations in which you want to include files more than once.
Why would you want to include a file multiple times?
Several reasons have been given in other answers (Boost.Preprocessor, X-Macros, including data files). I would like to add a particular example of "avoid code duplication": OpenFOAM encourages a style where
#includeing bits and pieces within functions is a common concept. See for example this discussion.
Ok, but why is it not the default with an opt-out?
Because it is not actually specified by the standard.
#pragmas are by definition implementation-specific extensions.
#pragma oncenot become a standardized feature yet (as it is widely supported)?
Because pinning down what is "the same file" in a platform-agnostic way is actually surprisingly hard. See this answer for more information.