DOIs are not URLs though. They are identifiers, as the name suggests. I don't think URLs were ever intended to be identifiers. At least that's not how they are used, as can be seen from the fact that there is a many-to-many relationship between URLs and pieces of content. Identifiers for published pieces that are independent from where that piece is currently published are useful. Just one example: An article can be published in many places and in many different formats (HTML, print, PDF, ...). The DOI allows us to establish that all these instantiations represent the same publication. You might say that we could use titles instead but these are not unique. We also can't use title + authors because authors change their names. And so on. DOIs therefore address a real need. That's why they've become so successful and omnipresent in academic publishing.