They lose focus because something newer or shinier catches their attention.
Also bcoz they don't have a better perspective and foresight. Anything that we do must be in the long run, the short-term things are just a subset of the long-run objective.
The main ingredient of foresight is purpose. When you don't have a defined purpose and when you don't keep a reminder of that purpose at frequent intervals or days, then you make a departure from that purpose unknowingly, when you engage and act with your purpose, you go deeper and you have a better foresight, then consistency will come into play.