I get in a funk and my brain refuses to push past it. I feel like Neo when they tell him about the Matrix and he pukes and passes out.
That is, there is a bit of a forcing function.
In any lisp compiler/interpreter suited for the Real World, the sexpr tree you see in your code isn't the datastructure the compiler uses. Why? Because that sexpr tree doesn't account for information like line numbers, which are needed by the compiler for error messages, and other data. The sexpr tree that macros are given is an abstraction.
As for error messages, macros make it hard to give good ones C programmers who think their preprocessor macros cause bad problems with error messages, get this: If there's an error inside a macro expansion, the compiler/interpreter is getting errors in code that litterally didn't exist in the source code in any form. Furthermore, there are the standard problems of line numbering and giving the programmer information about where the error was in the actual source, all caused by macros. And Lisp macros are far more pervasive in Lisp than preprocessor macros are in C.
Racket tries to solve this with syntax-case, but syntax-case is awkward and breaks the standard macro abstraction in a lot of ways. So the problem remains, and is fairly nasty. Although Lisps and Schemes HAVE gotten better about macro errors over the years...
Want to see that in action? Look at e.g. the intermediate language between Erlang source and BEAM bytecode - it reads like macroexpanded Lisp code with square brackets instead of parentheses.