The set of inserters for std::basic_ostream includes partial specializations for inserting char, signed char, unsigned char and such into basic_ostream<char, ...> streams. Note that these specializations are made available for basic_ostream<char, ...> streams only, not for basic_ostream<wchar_t, ...> streams or streams based on any other character type.
If you move these freestanding templates into the main basic_ostream definition, they will become available for all specializations forms of basic_ostream. Apparently, library authors wanted to prevent this from happening.
I don’t really know why they wanted to introduce these specializations on top of the more generic
template<class charT, class traits>
basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>&,
char);
inserter, but apparently they had their reasons (optimization?).
The same situation exists for C-string inserters. In addition to the more generic inserter
template<class charT, class traits>
basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>&,
const char*);
the library specification also declares more specific
template<class traits>
basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>&,
const char*);
and so on.