The reason is that the ICEL translators in the early 70s wanted to make this into a “greeting” rather than a “blessing.”
To put it into perspective, in the East, this is always a blessing. “Peace to this place/city/village/monastery/etc” [example "*mir vseem
"] while the priest makes the sign of the cross over the people. Clearly a blessing.
ICEL at the time wanted to make this into a greeting. They wanted it to sound specifically non-liturgical. They wanted it to be a greeting like we would do when meeting a friend while walking down the street. They wanted it to be a “Hello” or “good morning” or “how yall doin’?”
They still call it a “greeting” in the new English Roman Missal (at least, that’s what I recall from the samples because I haven’t actually had one in my hand yet). I do not know what the typical Latin language Roman Missal says about this.
Brendan: do you have one, and can you check?
In my Latin Missal circa 1950s there is no title given to this. It simply reads “the priest says” I’m not proficient enough in Latin to read the general instructions on the opening pages, so I don’t know if it’s called a greeting.
Anyway, that’s a very long way of saying that “And also with you” was an attempt by the ICEL translators to change the meaning of this liturgical action and make it into something non-liturgical and more secular.
In Latin (GIRM ¶ 46), the action is called
salutatio, a greeting or salutation. I’m not sure where some of your criticisms come into play, since ICEL was not responsible for the Latin text, nor were the drafters of the Latin working off an Eastern exemplar. And I know of no evidence that “Dominus vobiscum” is traditionally seen as a blessing in the Latin rite; you’d expect at least a sign of the cross or something if it were. Neither, I think, is there any evidence that ICEL wished somehow to make the greeting “non-liturgical and more secular,” but rather to avoid the clericalization inherent in a lopsided greeting, and to prevent confusion (Why peace merely with his “spirit”? What does that mean? Why not peace with all of him? What is the difference? Is it even
possible to wish “peace to the spirit” of a layperson? If so, would that be better or worse than wishing “peace to them”
simpliciter?). The result, as you well know, was a translation that was both unfaithful and bland, but there is no need to impute the motivations you seem to find.
Incidentally, the revised translation of the GIRM with American adaptations is online
here from the USCCB.