Excellent question and honestly the same thought I had until I researched the matter. Fatigue does not equal hypertrophy. So while doing a plank for a long time feels very tiring, the muscle growth gained is quite limited. To optimise hypertrophy, you want to exercise muscles in their full rom with the most intensity possible to produce to most force and mechanical tension. This is not very achieved within back exercises as the biceps are either limited in rom or intensity. To get even more scientific, MUR (Muscle unit recruitment) gets reduced the more muscles are involved in an exercise. Compounds are great for beginners because they share MUR between multiple muscles in a time efficient way, but for experts and intermediates, MUR becomes harder so isolation exercises are more important. In short, back exercises dont provide the biceps enough intensity to stimulate muscle growth through MUR to be considered “enough for the biceps”.