Present Perfect to express completion

If one searches in google about "in the past three years", the results are nearly always in the Present Perfect.
Ex: There have been modest gains in education in the past three years.....
Ex: In the past three years, we have achieved remarkable successes in protecting and enhancing the lives and.....

Why do we usually express a completed action in Present Perfect, even with the past time expressed?

If you want to distinguish English tenses based on whether
the action is "completed" or not, then you have to learn what
"completed" means in this context. It means something
quite different from the normal meaning.

An action which extends to the present time is _not_ considered
complete for the purposes of determining which verb form to use.

How many of the letters have you sent? (incomplete action)
I have sent eighty-five.

How many letters did you send yesterday?
I sent fourty-two.