I think everyone is seriously over-estimating how much it would take to land a legitimate starting pitcher. Almost every trade is completely lopsided in the favor of the the team getting the established pitcher.
The lone exception I can think of is the Sale for Moncada/Kopech trade over the past 10 years.
You're going to have to list examples to back up that claim.
If your basis is Cole, remember that Coles don't grow on trees, there's usually a lot of suitors when they are available, and the selling team might have specific requirements that may be difficult to satisfy, even if not requiring top prospects. (The Twins at the time didn't really have a Musgrove/Feliz/Moran package to offer, in terms of potential and MLB experience/readiness for the reloading Pirates. I could see the Rockies or Diamondbacks similarly looking to reload in a Gray or Ray trade, respectively.)
But off the top of my head, there are several recent trades which correlate just fine with the notion that the Twins may have to part with one of their top 3 prospects. Paxton cost a top-50 prospect last winter; Archer cost a couple recent top 20 prospects.
If you are talking rentals or 1.5 year players -- somehow the price was high enough to prevent the Twins from making a deal this past summer, despite a fairly clear need. And a lot of sellers have already sold -- Toronto, Arizona, etc. Are they selling any more? And do they still have anything left to sell?
Among younger or more under-the-radar types, there's Zac Gallen who basically cost a top-50 prospect. The other Marlins starters might be cheaper but might not be as good, and of course there's little urgency for them to deal either.
The Twins may be at a disadvantage right now too, because there aren't a lot of clear sellers (at least not a lot with good pitchers!) and a few of the most rumored sellers (Detroit with Boyd, Cleveland with Kluber/Carassco, and maybe even KC with Duffy) are in our division, which might complicate matters a bit (especially with Cleveland).