Here's the reason. Correa. He's here now and producing at a high level. Next year it's probably a better than 75% chance he isn't here. Buxton is healthy(so far). All of your young bats seem to be producing.
That said, I don't think taking a shotgun blast with all of our top prospects is prudent either, I would target one really, really solid bullpen arm, yep that may cost something decent. Then another bullpen arm that may be some sort of project, former starter, etc.
I honestly don't think going after a starting pitcher of high caliber will be worth it unless it is the type of #1-2 ish guy that we can have control of for a few years. If they can identify and pull off that sort of trade, I would be for using some heavy prospects for. That said, I don't want to go hard after a rental starting pitcher that will cost a pile, not worth it IMO.
If they are going to trade for a rental type starter, trade for a guy that won't cost you much in prospect value, but that can eat some productive innings whether that be as a starter, or a long man to backup guys like Bundy or Archer.