There is an interesting principle and ruling established by the Smak, the Tur, and the Rama:
One who davens and did not focus on his prayer, if he knows that he can pray again and focus, he should repeat the prayer, and if not, he should not repeat the prayer (Sefer Hamitzvos Katan***, Mitzvah #11).
This last opinion is expanded upon by the Tur and, in turn, by the Rama (Orach Chayim 101), who rule that should someone fail to have kavanah during the beracha of Avos, he should not repeat his prayer, because of the likelihood that he will not have kavanah the second time around, either.
That was way back when. But we still practice based on this today.
Why do we still assume, despite the enormous amount of Jewish education and the new heights of "frumness" that exists in many Jewish communities, that a person praying will not ever have proper kavanah and so does not repeat the shemonah esrei when their mind wanders?