Its not just the variability. Its the opporunity cost.
If we had 100 units of trade capital, we spent like 50 of them on the next 5 years. We don't know if we will be good those years. 1.5 WAR of good reliever is intensely useful when good and not so much when you aren't.
We could spend all 100 units this year or mostly this year, and be better this year. We could have three relievers, a starter and a good bat.
The whole thing is more sounds smart than actually smart to me.
Also, money is cheaper for us. In real terms if a team has half our budget, then we can spend twice as much as them for the same value. Prospects are the same price, which means i'd be more willing to spend money and less willing to spend prospects. That's essentially what the dodgers gm was talking about when trying to get as much business done in the offseason as possible. We seem to be the opposite. We love paying for multiple years of control at the exact most expensive time to do so.
The signings we should be making is from twins pitching dev. They obviously know what they are doing.