Nimzo: Why exchange here?

I've been looking through the Nimzo opening as part of the study plans that are on this site. I'm trying to make sure I understand the whys of the moves instead of just the moves themselves.

With the white queen in that position black no longer gets the result of doubling white's pawns with the exchange.

So why is the exchange still the correct move instead of retreating the bishop?

Yeah, there's your answer, in bold.

You weren't pinning the knight because you intended to inflict doubled pawns on your opponent.  You were pinning it because the pin prohibits e4.

That's why in lines that go, for example, 4.Nf3 (which allows an instantaneous doubling of the pawns, if black so chooses), black will generally respond with 4...c5, or 4...b6, or 4...0-0, or even 4...d5, all of which leave the pin intact.

It can't be that bad of a spot as it is the second most often played move in that position on the master games database of this site and scores well.

So essentially the big focus of the nimzo is on blocking white from e4.  If that is the case why isn't d5 played before castling more often (I see that it is the 3rd most common response). For me, playing d5 first seems to make a position I'd enjoy more as black.

I do see that when d5 is played before castling and then a3 is played (which is uncommon and that makes sense) that retreating to Be7 now scores good for black so that helps make it all clearer. Position after the d5 move first below:

Thanks for helping me understand it more.

In the first example, after BxN which is close to being forced, after QxB the most played move is b6 rather than d5. Personally think leads to very dull positions for black after Bg5etc. d5 is a more modern idea to speed up knights because lead in development is black's main compensation for two bishops in Qc2 lines.

In second position BxN is probably better too, because Be7 is leading to a QGD exchange position with useful extra a3 for white.

Think it is useful to have some knowledge of ideas of Queens Gambit declined because Nimzo is designed to be an improvement of it.