Why isn’t #pragma once automatically assumed?

There are multiple related questions here:

• Why is #pragma once not 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.

• Why has #pragma once not 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.