Also, there are programming languages where you can't use all the features. Yes, you can write your own version of printf() in C, but you can't write your own version println() in Pascal. Or your own version of PRINT in Fortran. This isn't meant as an excuse for why Lua is unable to use itself to the full extent, but to show that not all programming languages allow that.
Actually, following up on that, you can’t even do printf(). How do you do I/O? At the end of the day the core low-level functions in the C runtime MUST be coded either in assembler or using hidden compiler intrinsics to generate the necessary operations (such as raw OS calls).
—Tim
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