We use the Present Perfect to say that an action happened at an unspecified time before now. The exact time is not important. You CANNOT use the Present Perfect with specific time expressions such as: yesterday, one year ago, last week, when I was a child, when I lived in Japan, at that moment, that day, one day, etc. You CAN use the Present Perfect with unspecific expressions such as: ever, never, once, many times, several times, before, so far, already, yet, etc. Examples: I have seen that movie twenty times. I think I have met him once before. There have been many earthquakes in Japan. People have travelled to the Moon. People have not travelled to Mars. Have you read that book yet? Nobody has ever climbed that mountain.

The short answer is that you're describing the present situation by using a past event. This is what the grammar pattern indicates to us. For example, "I haven't eaten lunch" tells me that you are hungry now. We also use present perfect when talking about experiences (but without a time marker).