Why is he late for the meeting? Почему тут Present Simple и после why используется is а не does? What do you do on the weekend? Вот похожий вопрос тут используется do.

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In the first sentence, "is" is used because the question is asking for an explanation of a current or ongoing situation (his lateness for the meeting). Using "does" would be incorrect because it is only used for actions or activities that are habitual or repeated.

In the second sentence, "do" is used because the question is asking about a regular or recurring activity (what you do on weekends). This is why the Present Simple tense is used here.