Of the many things that God created before Adam, why mention plants, ground, and rain, again, at Gen 2:5?

Because the present narrative is about the creation of Adam, whose primary function was to “till the ground”; it is surely no accident that the name Adam resembles adamah, ground. In the narrative, neither rain, which was necessary for growing things, nor grass and plants, which could be grown, were needed, because Adam was not yet on hand to farm them. Of course, we have already been told in Gen 1:11-12 that on the third day God created plants, and presumably some time before that he first caused the rain to fall. But those verses are in no tension with Gen 2:5, because the narrator is simply reminding the reader that there was a time before rain, ground, and plants—and Adam—but that they will all, presently, be working together. This is to be understood as an origin story, in short, not just of man, but more precisely of man-the-tiller-of-the-ground.