Because C is a different language to SB. ScriptBasic is a programming language is it not? C is a different one.ScriptBasic is written in C. How is expanding it in C a different language?
ScriptBasic has a syntax and semantics defined somewhere I presume. Even if it's only actually defined properly as being "what the interpreter understands".
C is a programming language. It too has a syntax and semantics, defined by the C standard document.
These are two totally different languages. If I feed SB source into the C compiler it ain't going to work. If I feed C source into the SB interpreter it ain't going to work.
It does not matter that SB is written in C. That is an implementation detail. Any programming language can be implemented in any other programming language. SB could as well be implemented in Swift or assembler.
Large parts of the Rust, Swift and other language compilers are written in C++. They use the LLVM compiler back end. Despite the ease with which we can use C from Rust or Swift we make a distinction between the Rust, Swift and C languages.