Difference between sentences starting with "don't you...'' and "do(did) you..."

The combination of "Look, some buses are coming this way" which is an instruction (Look) combined with the fact that the buses are coming (and can be seen) at the present moment with "did you see them?" which is in the past tense means that the two parts of the sentence have verbs in different tenses.

This can be correct under some circumstances but this is not one of them in my opinion. "Look, some buses are coming this way, don't you see them?" is correct and does, as you say, express some surprise that the other person has not reacted to their appearance.

The forms which do not express surprise but just draw attention to the buses are "Look, some buses are coming this way, do you see them?" and the more common "Look, some buses are coming this way, can you see them?". In these cases the tenses are consistent.

The use of the negated form of the verb, can't (or in archaic speech "can you not" or "cannot you") in the first example expresses a degree of surprise or even incredulity that the other person does not perceive things as the speaker does.

Other similar forms are "don't you think that", "aren't you aware that" and so on. The form is also used to express surprise that someone lacks a skill, or posession; or that they do not enjoy or participate in some activity; or that they do not have a given status. For instance "Can't you swim?", "Don't you have a smartphone?", "Don't you play football?", "Aren't you married?".

The last four are different from the simple, neutral question which just asks for information without carrying a value judgement. The neutral versions of the last four are: "Can you swim?", "Do you have a smartphone?", "Do you play football?" and "Are you married?"

The tense of the verb has nothing to do with the degree of surprise, the same thing would apply to "Didn't you go to university?" (value judgement and surprise) and "Did you go to university?" (a simple request for information with no preconception or value judgment).