why dont they use present perfect continuous tense becase they mentioned time ? Why do they use present prefect tense ?

Present perfect continuous is used when the action is started in the past or present and still going on and we don't know when that action stops. Ex..I have been working as a teacher for 4 months. In the above example, the action started in the past and still continuing and we don't know when the action stops. Here time duration is long. Whereas, Present perfect tense is used when the action is just completed. It might have started in present or in past but it has stopped. No more continuation. Ex..I have worked as a teacher for 2 years. In the above sentence, there is no more continuation and the action is stopped. Here time duration could be short.