One explanation may be that the seal may be broken. Most windows have two panes of glass put together with a seal around the edges. If that seal breaks from age, weather conditions etc.. it allows air to get in between the panes. Since the air contains moisture it can leave behind what looks like drops of water. Sometimes it begins as a slight haze that gradually gets worse. There are some window companies that can repair this but many times the only option is replacing the window.