In the London system, white wants to keep their dark squared bishop,
Therefore after say
1. d4 d5 2. Nf3 Nf6 3. Bf4, black can play
3...Nh5, doesn't it break white's position?
[FEN ""] 1. d4 d5 2. Nf3 Nf6 3. Bf4 Nh5
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The bishop is pretty decent on g5 as well.
So after 1. d4 d5 2. Nf3 Nf6 3. Bf4 Nh5 4.Bg5 and now black would have to create serious weaknesses on the kingside to hunt down the bishop. But if he just returns the knight, he has lost a tempo.
[FEN ""] 1. d4 d5 2. Nf3 Nf6 3. Bf4 Nh5 4.Bg5 h6 5.Bh4 g5 6.Bg3 Nxg3 7.hxg3