Why does everyone say that after d4 nf6 c4 d5 cxd5 is good

@Ihavenothing
AFAIK 4. Nf3 is the current main move, followed by 5. e4.
After 4. e4, Nf6 5. Nc3 e5 is possible which can be avoided by 4. Nf3.
4. Nc3 g6 5. e4 Nxc3 is indeed Grunfeld, but I doubt that this is very popular, because the Grunfeld is considered to be a serious opening, while the Marshall Defence is considered to give clear advantage to white.

@Cruxo thank you very much , and thx to the rest.

Here is a high level game, where white prefers to transpose into known territory

tpr,that is a very good game.It is a high level game.I spent so much looking for moves that which makes sense. But isn't this game a win for black?

#25
You are right, black should win, but probably was out of time.
www.chessgames.com/perl/chessgame?gid=1697471

Summary:
After 1.d4 d5 2.c4 Nf6 the recommendation is
3.cxd5 because
3... Qxd5 runs into 4.Nc3 with the loss of a tempo and no sane way to enter the Grünfeld.
3... Nxd5 runs into 4. Nf3!
This is btw according to my knowledge and experience the only move putting hard questions to black. Now 4... g6 runs into 5.e4 and as there is no Nc3 Black has not the counterplay he wants in the Grünfeld. White is not winning yet but has the easier position.
Other black moves lead to White occupying the center with more space. Again Black has a more difficult game.

If you don't believe it you should try it against equal opposition in several games. It is a position to train space advantage in the center with two pawns against one pwan there.

#27
Occupying the center and having more space does not lead to a difficult game for black. Black can counterattack the overextended white center. This so-called "hypermodern" idea of 100 years old forms the basis of the whole 2...Nf6 idea. after 5 e4 black can retreat 5...Nb6 or even 5...Nf6 and counterattack the white center. The space advantage is overrated. Of course it is more difficult to play the black side, as black has to play accurately to avoid problems. a strong black player or an engine will show that the black position is OK despite the broad white center and the space advantage.