“Has he come yet'” vs “Has he not come yet?”

I referred to the Michael Swan's Practical English Usage, regarding the use of yet as an adverb (chapter 539.3 yet).

The following examples are given below:

• The postman has not come yet.
• Has the postman come yet?
• Is supper ready yet?

He explains that yet is used in the negative sentences and positive questions. He has not explained why it is so. To me “Has he come yet” sounds odd. Why can't we say “Hasn't he come yet?” instead of “Has he come yet?”

Is there grammatical base for this or is it just a matter of usage? If it is possible to use negative question like “Has he not come yet?”, how does it differ from “Has he come yet?”?

Could you please explain?

You can say both.

"Has he come yet?" is a simple enquiry as to whether the person you are waiting for has arrived.

"Has he not come yet?" implies that you think he really ought to be here by now.