Seat belts fixed to the seat have some advantages. However, until now, there is no specific FIA standard that regulates the construction of the seat to make it resistant to the loads transferred from the belt in an accident.
The reason why it’s connected to instead to the body, is that the roll-bar is integral to the whole body and it’s able to dissipate impact energy better with a properly designed cage. It’s also able to resist better than a threaded connection built into a body of a composite material. Additionally, until a few years ago it was much easier to simulate the dynamic behavior of metals compared to composite materials, making the predictability in the event of an impact “safer” and the level of protection more reliable.”