Why shouldn’t students rely only on spell-checkers while proofreading?

spell-checkers only catch errors that involve .
a. misspelled word
b. proper noun
c. homophomes
d. word capitalization
they can only tell if words are spelled correctly, not if they are used correctly. there is, in fact, no better strategy than proofreading by doing a close reading.
in particular,
a . using context clues
b. reading aloud
c. citing evidence
d. using "track changes"
is effective because it makes it easier to detect comma splices and run-on sentences.