sizeof() is a compile time operator. To calculate the size of an object, we need
the type information. This type information is available only at compile time. At
the end of the compilation phase, the resulting object code doesn't have (or not
required to have) the type information. Of course, type information can be stored
to access it at run-time, but this results in bigger object code and less performance.
And most of the time, we don't need it. All the runtime environments that support
run time type identification (RTTI) will retain type information even after compilation
phase. But, if something can be done in compilation time itself, why do it at run
time? 


On a side note, something like this is illegal...


printf("%u\n", sizeof(main)); 



This asks for the size of the main function, which is actually illegal:


6.5.3.4 The sizeof operator 
The sizeof operator shall not be applied to an expression that has function type....