Because everything is configurable and nothing is stored logically, its stored such that it can be pieced and parted out with a back end gui environment.
Have you looked around the site, or just at the postings? User profiles and settings, messaging, libraries and galleries? Notifications? Beyond that there is an entire administrative side you don't see. There is a reason why large forums use forum software, and don't just throw together something custom. Anyone can build a messaging board, it took years of lots of developers and community contributions to build something with as many features as vbbulletin.
Updgrades, or switching on integration features, and switching to a responsive theme (or a custom built one) would do all you are wanting without basically starting over... no lines of code. (oh, and IMO, its just that forums do not look modern, not that vbBulletin can't)
Be all for naught anyway, because anyone young enough to care about a modern look doesn't spend any time on forums anyway. Its a dying media.
*shrug* I've been doing Dev for 20 years, nothing is ever as simple as you think