If an AWPer pads out their stats by getting exit frags every time they save the weapon, that's nowhere near as impactful as someone like Xyp9x winning clutch after clutch with an AK/M4. Yet it's near impossible for Xyp9x to consistently outperform even the most average AWPers, no matter how many impossible clutches he wins. Does that seem fair to you?
Besides, the whole purpose of a rating system is to try and measure skill. If impact was placed higher than skill then we should see a lot more IGLs in the yearly top 20 lists.