Quote by Mendalla
They are basically building from scratch stuff that software like that (Xenforo, vBulletin, IPS, etc.) has been capable of for years
Not Invented Here, lol.
I've never seen forum software that does the quoting in quite the same way as here (not saying that's good or bad), nor the paste-a-link-over-the-text-to-hyperlink (which is unintuitive, but cool in a way, even though you can't easily edit the link after). But my exposure to forums has mainly been limited to punBB, FluxBB, phpBB and Vanilla. And I'm used to markup-based forums that use BBCode (shudders), Textile or Markdown rather than WYSIWYG style.
I expect (guessing) the sticking point for integrating an off-the-shelf system is licensing. None of the software you cited is open source, and runs proprietary code.
Open source forum software has also traditionally been a pain to customize. They always do something you don't want and you spend ages either searching for a plugin or have to find a way to hack it yourself.
I don't advocate DIY when there's good software already tried and tested, but maybe they have reasons for it.