[vicda]

Many understand the concepts perfectly well, but will still not always remember the arbitrary difference in naming. If memorization of arbitrary words is so effortless with repeated context then no one would mix up common words like effect and affect.

[wongarsu]

Effect/affect is mostly a problem for people with English as their first language: they learn the language by sound, and in pronunciation there (usually) isn't a difference between them. Same with "could of", "we're/were", etc.