In football, if touchdowns is the main way to get points, why can't the quarterback always throw the ball to the end zone every time?

To win at the game of American football, points are scored by touchdowns and kicking field goals, primarily. 
At a simplistic level, it might appear that throwing the ball to the end zone at every opportunity is the straight forward strategy. 

However, if one did that, then the defense would adjust and accordingly defend. Also, typically the team has to move the ball 75 to 80 yards to the end zone. Only a few strong armed quarterbacks can throw that far. Even fewer can do it under pressure. The other main issue is that there is a very low chance of completing the pass. 

So, smart teams and coaches, use a variety of strategies of both throwing the ball different distances, running the ball, and based on distance, kicking field goals. One of the key strategies is to mix it up, so that there is an element of surprise and the defense is caught off-guard.