'look after'/'look out for' have a close meaning. Why the former cannot be used?

[Frank Stern]

English Language Tests, Intermediate level

ESL/EFL Test #140 "Phrasal idioms with the verb 'look'", question 6

Airport authorities advised passengers to ......... suspicious-looking people passing through the international terminal.

(a) look for
(b) look out for
(c) look after
(d) look at

English Language Tests, Intermediate level

ESL/EFL Test #140 "Phrasal idioms with the verb 'look'", answer 6

Airport authorities advised passengers to look out for suspicious-looking people passing through the international terminal.

Correct answer: (b) look out for

Your answer was: incorrect
Airport authorities advised passengers to look after suspicious-looking people passing through the international terminal.
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"look after" and "look out for" have a close meaning. Why the former cannot be used here?

[Mister Micawber]

.
Not really similar.

Look after = take care of; care for.
Look out for = be vigilant against.
.
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Native English teacher at Mister Micawber's