Tripping and falling on the ground can hurt. Sprinting, getting kicked in the leg and then falling on the ground full speed can really hurt. Fact is that it's rarely fake, but fakers do happen. When you only get like 5 or so chances to score in a game and selling a call a little dramatically could mean the difference in getting a set piece that could lead to a winning goal, it's strategy. It doesn't excuse players like Drogba who clearly milk it, but if you do get legitimately fouled, making sure the ref sees it isn't a bad idea.