First of all, it is not a written rule and if players ran the route that their slide takes, they would be called out for interference. You have players who are reaching out with their arm for the base while their feet are some 7-8 feet out of the base path trying to prevent the fielder from turning a double play. You see slide after slide in which the runner goes way wide of the bag or far beyond it. It's not an attempt to avoid a tag; it's an attempt to prevent the fielder from completing a play and why that is not called interference, I have no idea. In high school and college baseball, the runner is required to slide straight into the bag. Why not in professional ball?