Because if you make 100 trades, and have 95 wins for 1 pip each, for a total of 95 pip gain, all it takes is 5 more trades of -20 pips to turn that into a losing run. (or losing account, depending on how much you are trading, heh)
My journal will be a perfect example of that when I next update it. I have been able to maintain a win rate of something like 70-80%, but not by choice. That is simply how it turns out to be. My problem is that my losers are bigger than winners over long-term, so it winds up being a loser stat.
Win percentage is just an artifact of trading, and not a method of it, in my mind. I just trade and trade and trade, and the win% turns out to be what it is. I do not look for it to be a certain number, I simply observe what it happens to be.