Jacob 5

Jacob 5:3 For behold, thus saith the Lord, I will liken thee, O house of Israel, like unto a tame olive-tree, which a man took and nourished in his vineyard; and it grew, and waxed old, and began to decay. I know it's an allegory, but even the Jews had to know that olive trees live a long time. There is a tree in Crete that was cut down and shown through tree-ring analysis to be slightly over 2,000 years old. (Source) Shouldn't matter to me, but it does a little bit.

The thing that interests me in this allegory is the idea that there is one tree that the Lord of the vineyard is concerned about. He grafts the branches of that one tree into other wild trees in other parts of his vineyard. But they're still his trees in his vineyard--why would he have wild trees in his vineyard? Why not prune and dig and nourish all the trees in his vineyard, rather than just grafting in branches and plucking fruit from only those branches, however they thrive?