People are still playing starcraft competitively, even 10 years after it was released! These days the gap between hardware and software is getting smaller and smaller, some games are brutal even on the best PCs. The reason the graphics aren't stunning is because sc2 has big shoes to fill and has to be an equally plausible competitive game thus they absolutely cannot afford to have any lag (plus they've probably had this in development for a very long time so the engine will be looking a tad dated now)