Without any access to system resources, you wouldn't be able to bootstrap from a tiny Lisp kernel to a richer system. A sector-sized Lisp (when you eventually get it down to that size) is a neat project, but something of a closed system.

Part of the magic of a minimalist Forth environment is that they come with primitives for accessing raw memory, the stacks, and facilities for extending the language. You can build new parsing words, interface with hardware, and so on.