为什么basic_ios没有移动构造函数

Question

basic_ios没有定义move构造函数有什么原因吗？ https://en.cppreference.com/w/cpp/io/basic_ios/basic_ios

protected:    basic_ios();
public:       explicit basic_ios( std::basic_streambuf<CharT,Traits>* sb );
public:       basic_ios(const basic_ios& ) = delete;