Because no one ever knows where the highest price will be and where the lowest price will be. Your purchases and sales are random points on the chart.
The unclear future of the market as you talked about is the major factor that compel people to go in loss by purchasing high and selling off at low. Plus one more reason is there and that is, no good planning on the part of investor or trader. Without a good plan or knowledge of the market, you are not able to survive in nay market. You should set some targets for yourself and when they are chased, sell off.