Maybe because as Red Sox fans, we are a bit spoiled by our excessive budget? If it means anything, Dave Dombrowski clearly shares your view about the trade value of minor leaguers. And he has used it to build several successful teams.
If the Sox deal away a prospect who works out elsewhere, they can fill the void with an expensive free agent. Bye bye Kopech and Buttrey. Hello, Eovaldi and Kimbrel. But not every team can do this. Think Pittsburgh misses Austin Meadows and Tyler Glasnow? How are they going to replace them? (Probably by dealing Starling Marte.) For some teams, those non-major leaguers are essential components of the future, and when they don't work out, bad things happen. But when you have a budget, that is a necessary risk.
And really, even when a GM does employ the Dombrowski method for a successful team, the eventual lack of those not-yet-ready-for-prime-time players can create several years of a weak performing and expensive franchise. Dombrowski has a habit of leaving teams in that state, as well...