I have been facing a lot of examples of "why didn't/haven't VERB" in English translated as 怎么不 VERB. Why don't they use 没 VERB?
According to the book "Modern Chinese Grammar" (http://m.sanmin.com.tw/product/index/000225652), we can roughly distinguish:
不: this word is a negation that comes with the speaker's subjective feeling, which means that the speaker might think the other should have done something (for example in the first sentence, you should have already got up)
没: this word, however, is a negation which is more or less objective. That is to say, the fact that the other person did not wake up is noted.
To sum up, in both sentences, the contexts show very strong meanings that both speakers did not agree with what the others have not done, and therefore the negation word "不" was used.
As a native speaker, I feel that using the negation word "不" is usually more powerful and that implies the relation with the other is more close. Hope this answer can give you some clues.
You're right, it's quite fine to use
没+V is usually used for the past tense.
你怎么还没起床？ => Why haven't you got up? (The intent is, you should have got up early.)
不+V could be used for past tense, present tense or future tense, according to the relevant context.
你怎么还不起床？ => Why don't you get up? 你怎么不去学校？ => Why don't/won't you go to school?
In your examples I honestly can't see any essential distinction, they are interchangeable here.
Speaking about the tense, 没 may intend to express a very weak comparison between the past and the present(before you didn't, now you still haven't), so I will naturally translate it using a prefect tense, but basically either past tense or perfect tense is ok for them both.
没 is not a verb because there is no action in 没，nothing.
不 is not a verb neither because there is no action in 不，not.
The verb are 起 and 选:
已经九点了，你怎么还不起床？ (起 is the verb here, it means "get up")
你怎么不选一个更好的作文题？(选 is the verb here, it means "choose")