I personally would disagree that it gives better readability.
Also, C macros were not designed for such purposes.

The reason is simply writability (fewer keystrokes).

One of the main problems of C/C++ contest macros is that everyone has their own set of macros, so writes in their own unique language. You may be fine with that as a lone contestant who never reads other contestants' code (no team contests, no hacking attempts, no problemsetting, etc.). But as you reach out to these and other areas, you find yourself reading other programmers' code quite often, and learning a new set of macros every time is just not worth it.

Another problem is applicability of macros: different versions have different domains, and with a particularly unsafe tool such as macros, stepping outside the domain usually results in a silent error. Considering the time one has to spend to debug a C preprocessor macro, the usability gain of macros is questionable.