Because it's a standard pitfall: https://gcc.gnu.org/onlinedocs/cpp/Duplication-of-Side-Effec...

If the strcp() macro is used with a function as the first argument (or an expression that has side-effects), it's not obvious at the call site (or, probably, intended by the programmer) that the argument will be evaluated twice.

E.g. suppose we write something like

    char* buf = strcp(((char*)malloc(4)), "foobar", 4);

expecting this to copy the beginning of the string "foobar" into a newly-allocated 4-byte buffer. Oops... this will actually call malloc twice (leaking the first buffer), and it won't have written the intended '\0' into the buffer that actually ends up getting used, so all bets are off...

If strcp were a function, its first argument would be evaluated just once, and it would work as intended.