Because "communicating with messages" is one of the important features of objects, but objects are the focus.
The confusion, I think, mostly came about because in the late 1980s early 1990s, an explosion of "Object-oriented" programming happend by gluing classes into existing popular structured programming languages -- producing ObjectPascal and C++ which became the popular "OO" languages which became what "OO" meant to lots of people (Objective-C was a contemporary, and despite being C-based like C++ was more like Kay's idea of OO than C++, but didn't become popular until later.)
So what became popularly conflated with "object oriented" programming was statically-typed, class-oriented programming typified by C++ and similar languages (e.g., Java.)
There are certainly ways (often not the same ways between languages) in which newer languages -- Erlang, Python/Ruby, etc. -- are more "object oriented" than Java/C++-style languages.
The confusion, I think, mostly came about because in the late 1980s early 1990s, an explosion of "Object-oriented" programming happend by gluing classes into existing popular structured programming languages -- producing ObjectPascal and C++ which became the popular "OO" languages which became what "OO" meant to lots of people (Objective-C was a contemporary, and despite being C-based like C++ was more like Kay's idea of OO than C++, but didn't become popular until later.)
So what became popularly conflated with "object oriented" programming was statically-typed, class-oriented programming typified by C++ and similar languages (e.g., Java.)
There are certainly ways (often not the same ways between languages) in which newer languages -- Erlang, Python/Ruby, etc. -- are more "object oriented" than Java/C++-style languages.