It's because it's an allocation exercise. The amount of practice time available in a given week is a fixed amount. So a big part of Cal's, or any coach's, job is determining how they will allocate that time. Any minute you spend on rebounding is a minute you can't spend on free throws or on defense.
So to decide how to split this, it becomes a matter of deciding where you think you'll get the biggest return. In the case of free throws, you'd have to ask yourself questions like:
- By how many percentage points do we need to raise our average for it to make a difference in our wins and losses?
- Then you need to ask yourself how much practice time will be required to achieve that increase
You'd ask the same types of questions for other aspects and then start to structure your practice accordingly.
The problem with free throws is that it takes a very significant amount of practice to raise your percentage a small amount. In that same amount of time, you could've made much greater relative strides in rebounding, defense, etc.
In your example of driving, this focus increases the likelihood that a guard will be shooting free throws. Guards (Briscoe excepted) tend to shoot free throws better. You also gain the benefit of putting the other team's players into foul trouble, which either forces them to be less aggressive or put their best players on the bench. This helps your offense and defense later in the game.
At that point, are you better off spending time practicing free throws or working on defense, rebounding, offense, etc? Given the way those factors impact the game, and the amount of effort it takes to improve them, you're better off not spending too much time on free throws.