Why is earwax considered a barrier (for ablution/ghusl) even though it originates from the body, while dried sweat is not considered a barrier despite also originating from the body?

It doesn't matter whether the barrier originates from the body or not; what matters is whether it prevents water from reaching the skin. Eye discharge has a substance and must be removed, but sweat does not. Earwax, if inside the ear, does not need to be removed. However, if it appears on the outer ear canal, it is considered a barrier, and some jurists, like the Shafi'is, hold that its removal is obligatory. The Malikis, however, do not consider washing the outer ear canal obligatory at all because they consider it part of the inner body.