It's simply a matter of translation. If you look at a Douay-Rheims Bible, a NRSV Bible, a NAB Bible, a Jerusalem Bible, and a Knox Bible, you will notice that each is slightly different, but they all convey the same meaning. Also, if you look at the prayer in the different Gospels even within the same translation, you will notice that they are slightly different.
We use the translation that we use simply by centuries-old custom. It is very familiar to Catholics (and most Protestants as well), and it matches the meaning of the original Greek very well, so there is no need to correct it.
A priest in my diocese recently gave a very good, thorough answer to the question of why the text of the Lord's Prayer was not recently changed on his parish's website:
Let me begin by pointing out that the use of vernacular (for us that is English) within the Mass is relatively new when compared to the larger history of our Church. For centuries the liturgical prayers were said in Latin. This however, has not been the case for the bible. Rather the art of biblical translation has an extensive history. Already in the 4th century the ancient texts of Greek and Aramaic were translated to Latin, as history tells us by St. Jerome. Now fast forward to the 14th century where John Wycliffe translates the bible into English based on the Latin text.
Why this history? Well, the Lord’s Prayer is a direct quote from scripture, and the translation of scripture from the Latin to the English had already begun in the 14th century. Hence, for scripture there is already a long history of working with the text, examining the text, all to convey its meaning in vernacular languages (i.e., English). That is why we have so many different versions of the bible, and in the end translation is an art. As it stands the translation of scripture -- this includes the Lord’s Prayer -- has gone through centuries of translations, where the text of Mass has gone through decades. Thus, the text of Lord’s Prayer, simply put, is fine. Its translation from Latin to English is adequate and therefore there was no need to re-translate it in the Third Edition of the Roman Missal.