The Lua developers have steadfastly refused to sanction even the most basic extension packages by inclusion in the Lua distribution. Until quite recently [1], they have even stuck to the point of view that applications should supply their own C host program.
As, of course, LuaTeX does. LuaTeX also packages a number of extensions that have wider utility than document creation, like lfs, socket, lpeg etc. The temptation is therefore quite strong, especially when developing platform-independent software, to bypass the usual channels of package loading and rely on texlua as one's standard Lua interpreter. [1] The Lua 5.3 manual was the first to say "Frequently, this host is the stand-alone lua program" instead of "The Lua distribution includes a sample host program called lua".
