The verb "pretend"

Hi, this is Apple, who still can't log in under my former username.
Anyway, here is my question.
Look at the following sentences.
I know (2) is not correct while all the rest are good.
I know that as-if clause can not follow "pretend".
Is there a plausible explanation for this, or do we just have to learn it individually?

(1) He pretends to be ill.
(2) * He pretends as if he were ill.
(3) He pretends that he is ill.
(4) He looks to be ill.
(5) He looks as if he were ill.

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