Greenspun's law, "Any sufficiently complicated C or Fortran program contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of Common Lisp." is really just a special case of a general principle. There's a lot of cases where if you use the wrong tool and/or don't know the right libraries, or don't or can't use them, or just won't, that you'll reinvent something that already exists, badly.