Everybody does.  It's because all the fair-weather friends jumped ship when we had a run of bad luck, and are now climbing back on board (seemingly unable to comprehend that finding several blocks in quick succession does not guarantee that more will soon be found) and diluting the value per share of any block found.  If luck remained stable at 100%, it would mean that more blocks are solved, offsetting the smaller reward, but of course real life ain't like that.