But it's not a skill unique to strikers. There are keepers who've been designated penalty takers. Heck, for the Chelsea game, why didn't Harry take the penalty? It doesn't have to be the striker's job. Hence, it's not relevant to evaluating them when you're trying to find the best guy for scoring from open play.
We're looking for strikers. We don't need a guy good at penalties, we need a guy good at scoring from open play. So you strip the penalties out of the totals to get numbers more accurate to what you look at.