Kernel cannot be reasonably compiled because compound operatives can depend entirely on user input data. For example, the presence of string->symbol means that

Kernel cannot be reasonably compiled because compound operatives can depend entirely on user input data. For example, the presence of string->symbol means that a user could insert a custom operator into an environment at runtime then evaluate the expression to which it refers. There's now way to account for this at some theoretical 'compile time', unless you're JIT compiling after every external input, which I'm pretty sure is what we'd call interpretation.