Opening theory: Semi-Slav and side lines

This position is after the best moves for Black and White from 1.d4 d5 2.Nf3 Nf6 3.g3 c6 4.Bg2 - a continuation that is not used by players above 2600.

This position is easily archieved from the position above if you memorize minimum amount of opening-theory. It`s an easy draw.

Instead of Qb3 like in the first diagram White goes here Bb2. Again, with minimum knowledge of theory it`s an easy draw. These two continuations(Qb3, Bb2) are the best from 1.d4 d5 2.Nf3 Nf6 3.g3 c6 4.Bg2. The rest is even mòre drawish.

Here comes the winner:

While this position is the most ambitious way to continue after 1.d4 d5 ..( I don`t write moves to reach this position since everybody knows it).

My questions are:

1. Why does people play "1.d4 d5 2.Nf3 Nf6 3.g3 c6 4.Bg2" under 2600 fide?

2. Why does people play "1.d4 d5 2.Nf3 Nf6 3.g3 c6 4.Bg2" above 2600 fide but only in Blitz?