There are all sorts of reasons for that, but for explanation, you need look no further than the Florida Marlins.
With the recent trades of Cameron Maybin and Andrew Miller, the two highest-profile players they received in return for Miguel Cabrera and Dontrelle Willis, the Fish essentially conceded defeat on that December 2007 deal. Although Florida unloaded Willis' unwieldy contract in the trade, nothing it received from Detroit made up for the loss of a player like Cabrera.
When and if the Royals look to unload Zack Greinke, or the Brewers consider moving Prince Fielder, they must be careful, but also aggressive. And they should learn from Florida's experience.
Get a package of nothing but high-end prospects, and you might strike gold, but you might strike out. But at least you have the chance at something big -- like the Indians scored when they dealt Bartolo Colon in 2002 for Brandon Phillips, Grady Sizemore and Cliff Lee. Get a flotilla of low-risk prospects, and you're unlikely to receive anything of real long-term impact for a signature player.
Get even one young, established stud ... now you're talking.
While the trades of Maybin and Miller have been framed as an argument against trading veteran talent for youth, it's really not that simple. Replenishing the farm system can be a great move, as long as a club commits to getting real talent in the deal. The Marlins didn't quite do that, getting two high-risk, high-reward players mixed with four low-risk, low-reward players.
The bigger problem may have been that the two top prospects already showed danger signs. Maybin was promoted far faster than he should reasonably have been. Miller had consistently showed a worrying lack of command in his Major League career.
So the lesson to be learned from the Detroit-Florida trade isn't strictly a caution against getting prospect-happy. It's also a matter of which prospects you choose. The Marlins elected to take quantity over quality, fleshing out the deal with four middling prospects rather than one more high-impact potential star. Dallas Trahern was a 34th-round Draft pick, a right-hander who was coming off a fine year at Double-A but who didn't have the strikeout rates to get you excited. Burke Badenhop was a 19th-round pick with a somewhat similar profile.
Eulogio De La Cruz and Mike Rabelo had both reached the Majors at the time of the trade, but neither had given any indication that he was a potential star. They were warm bodies, but not the kind of return you must receive when you trade a transcendent player such as Cabrera.
"When we made the trade, it was a big trade, absolutely, at the time," Marlins president of baseball operations Larry Beinfest said recently. "We were hopeful that Andrew and Cameron would be big parts of our ballclub for years to come. In both their cases, they wished and we wished their performance would have been better at this time."
But again ... better still is to get young cost-controlled Major Leaguers. Those are the guys you want. That's why everyone is knocking on the D-backs' door about Justin Upton and calling the Cardinals about Colby Rasmus. We already know these guys can play big league ball. They're young. They're cheap. And if you trade for one of them, you'll have him for a long time.
That, of course, is why it's hard to get those guys. Teams aren't just giving them away. But for a club with an in-demand veteran star such as Greinke or Fielder, those types of players must be the starting point. At the very least, "Major League-ready" should be a minimum. Pitchers in A ball, no matter how exciting their arms, won't do the trick, as the Indians learned in dealing Victor Martinez to the Red Sox and Lee to the Phillies.
It's a tricky line to walk, but that's to be expected. These are the sorts of deals that shape a franchise's direction. A front office must get them right. And you can win deals where you're a seller. It's just not as easy or as automatic as it sometimes appears.
In a way, the Marlins do seem to have learned from the Cabrera experience. And in a way, it's clear they didn't learn a thing. In trading Dan Uggla for Omar Infante and Mike Dunn, Florida did receive one risk-reward prospect. Dunn throws hard and is left-handed, a rare and intriguing combination, but he's also a relief pitcher -- which limits just how much value he can have in the long run.
But in making Infante the centerpiece of the deal, the Marlins bought high on a player with very limited potential for improvement. They traded a player whom they controlled for one more year, for another player they'll control for one more year. Infante is just the sort of player you flip for young talent -- and that's the one way it's easy to see the Marlins actually winning their most recent deal in the long run.
Matthew Leach is a reporter for MLB.com. Read his blog, Obviously, You're Not a Golfer and follow him on Twitter at @MatthewHLeach. This story was not subject to the approval of Major League Baseball or its clubs.