As the strong nuclear force is a short range force, and as it is way stronger than Coulomb force, Lumen Learning answered my question as follows: “for low-mass nuclei, the nuclear attraction dominates and each added nucleon forms bonds with all others, causing progressively heavier nuclei to have progressively greater values of BE/A.” But according to wikipedia:
“a neutron has a root mean square radius of about 0.8 fm,” and “the root mean square radius of a proton is about 0.84–0.87 fm.” which means that the diameter of a nucleon is definitely less than 2 fm.
“the nuclear force is powerfully attractive between nucleons at distances of about 1 fm but it rapidly decreases to insignificance at distances beyond about 2.5 fm.” which means that the strong nuclear force of a nucleon -surrounded by nucleons- has an effect only on nucleons next to it, and it doesn’t really have any significant effect on nucleons beyond those, i.e. a nucleon doesn’t really form bonds with all other nucleons in medium-mass nuclei. So, why does the BE/A rise with the rise of A, then start dropping away.