Because I don't think it does. At least not for the decade of campaigns I ran. Maybe the way other people did it became a required part of the game. And as is evidenced by many threads about character building there is a group of players that love to do this. But at my table characters always grew organically. At 6th level the Barbarian took Toughness because he felt the character had shown toughness the previous level. It is not the optimal thing to do, but that's not the way we play.
But maybe back in 2000 when you read the PHB the first thought to you was to pick out all the character choices from levels 1-20 and plan everything out ahead of time. Different people read a book, they get different things out of it. Goodness knows I never got anything out Ayn Rand's Fountainhead like my Literature teacher did.