Because that's why it exists. It's about GMs gathering more data, not because they need it, but because it makes their jobs easier. You're trying to tell me why it's a necessity, but it's not; it's a luxury that they're portraying as a necessity so people don't complain.
If you think that's my point about Europe, you completely missed it. I'm saying that the thought process you mentioned--where a GM drafts Kwame Brown because he's in HS, so could be Amare--is a stupid thought process. Assuming the best HS player in the draft is the next Kobe is just as dumb as assuming the best college player "could be" Michael Jordan, just because both went to college. Or assuming the best European player is the next Dirk, just because he's European. Drafting players based on the line of thinking you offered would be incredibly dumb. Fortunately, I don't think most GMs (or any) actually think that way, and that you mostly just made it up.
For the trillionth time, saying they're "tested" in college, and not before, is based on absolutely nothing but your own personal fancy. I could easily argue that every single first-round pick ever is financially set without being tested.