math.MaxUint64 is a constant, not an int64. Try instead:
fmt.Printf("%d", uint64(num))
The issue here is that the constant is untyped. The constant will assume a type depending on the context in which it is used. In this case, it is being used as an interface{} so the compiler has no way of knowing what concrete type you want to use. For integer constants, it defaults to int. Since your constant overflows an int, this is a compile time error. By passing uint64(num), you are informing the compiler you want the value treated as a uint64.
Note that this particular constant will only fit in a uint64 and sometimes a uint. The value is even larger than a standard int64 can hold.