"Why don't they help him?" Why do we use simple present here?

The simple present tense in these sentences refers to the general time of "now." The speaker is indicating that an act of help would be appropriate at the present time.

The present continuous in these sentences refers to a more specific, ongoing time period. The speaker is indicating that an act of help should be actively occurring at the exact time that the speaker is asking the question.

Thus:

Why don't they help him?

• Interpretation: I observe that there is a person in need of help, and I observe that at the present time no one is helping him. I expected that someone would have helped him by now, and looking around I don't observe anyone rushing to help him in the very near future.

Why aren't they helping him?

• Interpretation: At this exact moment, I expect someone to be actively engaged in helping him. Perhaps he was being helped in the recent past, and perhaps he will be helped in the very near future. However, at this exact moment, I observe no one actively engaged in helping the individual.

Both are accurate (grammatically), and they are close enough semantically to be all but interchangeable. But I think that the first sentence implies that no help has come or will come (unless the situation changes, perhaps because the speaker is asking the question). The second sentence implies only that it's not happening in the present; the listener cannot infer anything about the period of time immediately before or after the speaking event.

If someone had helped the man and then left, the first sentence would be confusing, and the second sentence would be more likely to convey the speaker's intent (i.e. Why aren't they still helping him?).

On the other hand, if someone had promised to help the man and hadn't arrived yet, the first sentence would be a more general call of despair rather than a critique of the potential helper (i.e. Why isn't anyone at all helping?). The second sentence would be a rebuke regarding the fact that the potential helper had yet to arrive (i.e. Why isn't he/she already here to help him?).

We use 'simple present tense' for what normally happens, what is natural to happen and for actions in the present time or for habitual or eternal truth.

• Wind blows.

• Flowers bloom

• Birds sing.

• Man helps.——— all these are natural, normal and expected of the doers to behave as such if otherwise nothing untoward happens. So, when men desist from what normally expected of them, we ask the question in simple present. We cannot use 'present continuous' as no one has so far come forward to help. To my mind, so long as help is not forthcoming, we would go on using simple present tense.

English language is very much practical. That is why it says, "I am better, not good". In my mother tongue, I use 'good' and 'present continuous' in a situation as in the post above.