I bet they never covered how to create a robust archive and indexing system for everything you write, either? And that leads to the self-plagiarism problem, too.
If everything you write is properly documented and archived, then you have the tools you need to do the due diligence on your past writing.
But how many college students even back up their computer? Much less keyword their documents. Or create indexes of their writing - both final papers and incomplete work - that they can use to find things later?
We are smart monkeys, but without proper training, all our tools are glorified hammers.