Because "react" is difficult to search for due to all the "reaction videos" and related nonsense. I agree it's a bit odd to use it in a name like that but more

Because "react" is difficult to search for due to all the "reaction videos" and related nonsense. I agree it's a bit odd to use it in a name like that but more than once I've had to add "js" as a qualifier when trying to google for something React-related. I'd imagine for some people this becomes so reflexive they simply call React "React.js" to avoid ambiguity. Adding or dropping a "js" suffix to library names like that is actually pretty common throughout the entire JS ecosystem. Many library officially call themselves "<name>.js" but then go on referring to themselves without the suffix throughout the documentation and release their packages that way too. I recall something similar in Python where libraries would frequently be called "Py<name>" or "<name>py" and people would sometimes add the "py" affix even when the library didn't actually have one in its name to clarify they mean the Python library of that name rather than something else.