Move orders in the 6.Qc2 Anti-Meran Semi-Slav After 1.d4 d5 2.c4 c6 3.Nf3 Nf6 4.Nc3 e6 5.e3 Nbd7 6.Qc2 Bd6 White can play the so-called Shabalov-Shirov Gambit‎ with 7.g4!? on the grounds that 7...Nxg4 allows 8.Rg1 and the important g7 Pawn is going to fall. Of course this is not a forced win by White, but this Gambit puts certainly some pressure on Black. 

My main question is: why is Black always playing 5...Nbd7 and 6...Bd6 (or in reverse order) and not 5...Bd6 and 6...0-0, after which 7.g4 could be more safely accepted  with 7...Nxg4 (the g7 Pawn is no longer en prise after 8.Rg1)? Is there any particular reason for playing 5...Nbd7 or 6...Nbd7 before castling?
A lot of opening play is about what you are trying to avoid. While sharp, black does quite well against 7.g4. In reality, the quieter lines seem to have much more venom if the figures in Mega 2020 are to believed.

In addition, with the move order you mention, white can get e4 in very early, or can play the Karpov line among other choices. The main line I am giving scores at 73.4% compared to only 56.2% for Shirov's g4. I think that g4 was a great surprise weapon, but now, it has been analyzed sufficiently with computers, and it no longer the great threat it once was. Black can respond with the best move, h6 instead of taking on g4.

 [FEN ""]

 1. d4 d5 2. c4 c6 3. Nf3 Nf6 4. Nc3 e6 5. e3 Bd6 (5... Nbd7 6. Qc2 Bd6 7. g4 {The Shirov Gambit.} h6!) 6. Bd3 (6. Qc2 O-O 7. Be2 Nbd7 8. O-O {Karpov's favorite line.}) 6... O-O 7. e4 dxe4 8. Nxe4 Nxe4 9. Bxe4 Nd7 10. O-O Nf6 11. Bc2