This means that code and data is typically hacky, unpolished, poorly documented, unmaintained, and unsuitable for public use (or often not publicly inspectable at all). Often the folks doing the grunt work are graduate students, who leave after a few years, leaving a new batch of graduate students to deal with the code without much guidance, so it tends to accrete features but seldom get any refactoring love. Nobody is paying for bug fixes or software maintenance or documentation.
Someone coming from industry (or even just a tinkerer) who wants to use the same idea in a production environment often realizes halfway through reimplementation that it isn’t actually better than existing alternatives, and that the analysis/benchmarks in the paper were limited or misleading for a general context. Sorting the wheat from the chaff can be pretty demanding, ideas can sometimes be obfuscated by abstruse mathematical formalism, and academic researchers don’t typically go out of their way to make their output usable for non-academics.