It's the English version of the subjunctive mood. In this case, it's the past subjunctive, which expresses a counterfactual conditional situation—one that you wish were true but isn't.
In a lot of other languages, it would be much more apparent that you were writing in a different mood, because the verb would be conjugated differently. But that's not really how English rolls.
So the sentence "Why not read a book?" isn't in the past tense because it's not counterfactual; it doesn't refer to any hypothetical, non-existent version of reality.
(References from Wikipedia.)
(If you're interested, I found this earlier question
on EL&U, which has some interesting information in the answers on the subject of why
the subjunctive is so hard to notice in English.)