The report-result function needs both the source code and the result of the execution.
The macro CHECK provides both from a single source form.
If you put a bunch of check forms into the file, they are easily compiled using the usual process of compiling Lisp files. You'll get a compiled version of the checking code.
Using a function and EVAL (better use COMPILE) you would have deferred the source evaluation to a later time. It would also not be clear if it is interpreted or compiled. In case of compilation, you would then later get the compiler's checks.
