DisplayPort specifies up to four lanes of display data but also an auxiliary data lane that can be used for anything, including USB or ethernet.
AFAIK the DisplayPort 1.2 aux lane only has enough bandwidth to accomodate USB 2.0, so any USB 3.0 hub would need to connect through a dedicated cable. As you've noticed though, in practice the aux lane is rarely used for anything beyond basic management commands for the display and such. Special functions like ethernet or USB would need support from the graphics card and drivers in order to route data through the GPU's outputs. So, if it were used it would most likely be found on laptops or tablets. But, there have been developments since the DisplayPort specification was designed, and with the introduction of USB Type-C with DisplayPort Alt Mode, the DisplayPort aux channel feature is effectively obsolete and I doubt it will ever see a move from the industry to implement it. Instead, we will see the opposite, DisplayPort carried through USB ports