Well, the reason for this is fairly obvious. If you are an average defensive catcher who hits 15 HR a year, you are very valuable...more valuable than a 1B with average defense and 15 HR. If you ranked prospects on bat alone, it would be all 1B and LFs.
How did we make the value of Albies vs. Freeman a simple determination of who has more pop in their bat? Because 1B is loaded with bats at the ML level, you have to really rake to have much value there, especially if you don't offer anything else.
Freeman was really good two years ago and has been worth about 2-3 WAR his other years, on pace for about 3.5 WAR this year. He's good to very good and still may improve. But why would anyone think it's crazy for Albies to eventually become that valuable a player at the ML level?
Of course Albies won't hit 20+ HR. If that's your sole determination of a prospect or player, then sure, Albies will never be Freeman.