Well as a begginner I always questioned this. However over time I thought of it and when I broke down how spells work I figured it out.
The Answer is Symbolism. Symbolism is what helps spells focus stronger (however it becomes less neccassary the more expierenced you get at energy manipulation). What does symbolism have to do with why custom spells work better?
Symbolism is a personal thing. Your symbolism is differant than someone elses. So if you use someone elses spell you are using a symbolism that isn't like yours. That creates a gapse of Will, and understanding. This weakens the effect.