We read the original works because the underlaying systems and logic of capitalism haven't changed since the 1800s, while in the intervening century and half we've had a concerted effort by literally every capitalist friendly org on the planet mystifying and distorting the science for political ends, leading to alot of inevitable backtracking and dead threads
Also if you're looking for a modern distillation of Marxism, Anwar Shaikh's Capital is the resource you're looking for