Comment by DaniFong

You're right that if you keep the implementations of Lisp compile time macros and use only that, you have as hard time reducing code that's already been macroexpanded, or applying global effects.

But when you might macros and the language itself, what you find is that you already have the pieces of what you need for a serious compiler: a symbol table, built in, a way to manipulate the parse tree, an easy way to do local expansions, and most importantly, a fully featured language.

You can do this with existing Lisps by a a few methods: making first class runtime macros, for example, or by saving the source code and working over it in passes.

Does this explain it?