Why does flowing off the end of a non-void function without returning a value not produce a compiler error?

Ever since I realized many years ago, that this doesn’t produce an error by default (in GCC at least), I’ve always wondered why?

I understand that you can issue compiler flags to produce a warning, but shouldn’t it always be an error? Why does it make sense for a non-void function not returning a value to be valid?

An example as requested in the comments:

#include <stdio.h>
int stringSize()

int main()
    char cstring[5];
    printf( "the last char is: %c\n", cstring[stringSize()-1] ); 
    return 0;


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