Because that won't change by year 4 of these young pitchers' tenures.
If they keep it up, they'll get paid like the elites they are. And if they don't, then pretending they are because of 1-2 good years is failthought itself.
I mean sure, if you can luck out with a rookie to get the job done, rock on, but let's not pretend that rookies are predictable. For every Papelbon, who goes on to have a stable high level career, you have 2, 3 guys who don't. And even the guys that do, eventually get paid like guys that do. So if you want to throw away your productive, steady elite RP just because you have to pay him what he is, and go back to the grab bag, don't be surprised if it bites you in the tail.
Basically it comes down to what you're saying here: I want the best, but I don't want to pay for it. One of these two statements is going to have to give way, either you pay for the best, or you settle for less than the best -- especially because "the best" is frequently not available for any money at all -- as is the case for us right now.