Looking at this through the intentions of the moves helps me make sense of this.
1. d4 establishes a dark-square pawn in the center. Light-square control however is still missing. If possible, White will remedy this with an immediate
2. e4 but one must wait for Black's reply.
1. d4 d6 there's no immediate challenge and White can take the central light-squares with
[FEN "rnbqkbnr/ppp1pppp/3p4/8/3PP3/8/PPP2PPP/RNBQKBNR w KQkq - 0 2"]
This controls both the light-squares and the dark-squares and Black must attack the center. If Black does not fight-back then Black will be disadvantaged and may even lose the game.
1. ... d5 is a fine response. It takes the central light-squares but there is an interesting downside. Somehow White's dark-square control has increased too - Black's pawn on d5 can never retreat to d6 and support a dark-square break:
[FEN "rnbqkbnr/ppp1pppp/8/3p4/3P4/8/PPP1PPPP/RNBQKBNR w KQkq - 0 2"]
In fact, White has the dark-squares all wrapped up suddenly, why not challenge the light-squares with a Queen's Gambit?
[FEN "rnbqkbnr/ppp1pppp/8/3p4/2PP4/8/PP2PPPP/RNBQKBNR w KQkq - 0 2"]
1. ... d5 works, but
1. ... Nf6 is a different try. Note that this move also controls the light-squares in the center. It also doesn't commit the d-pawn. I am not a master but I feel like this is why
1. ... Nf6 is said to be more flexible.
[FEN "rnbqkb1r/pppppppp/5n2/8/3P4/8/PPP1PPPP/RNBQKBNR w KQkq - 1 2"]
1. ... Nf6 White still wants to at least make a try for the light-squares. And why not? Just like in the Queen's Gambit, we have the dark-squares wrapped up, let's get the light-squares too.
The trouble is that the Knight controls
e4 - well, no problem, we know we can play
[FEN "rnbqkb1r/pppppppp/5n2/8/2PP4/8/PP2PPPP/RNBQKBNR w KQkq - 0 2"]
Game on - and as you say, there's nothing wrong with getting to this position via the
1. c4 Nf6 2.d4 move-order. But remember our point about
1. ... Nf6 - that it was more flexible? Well, so too are there some down-sides to
2. d4. Notice, for example, that the dark-squares aren't under-threat. Maybe we could get that dark-square control by developing a piece:
[FEN "rnbqkb1r/pppppppp/5n2/8/2P5/5N2/PP1PPPPP/RNBQKB1R w KQkq - 2 2"]
This also reserves the right to play
d3 later, possibly supporting the
e4 square. Imagine this:
[FEN "r1bqkb1r/pppp1ppp/2n1pn2/8/2P1P3/3P1N2/PP3PPP/RNBQKB1R w KQkq - 0 4"]
Or you could go full hyper-modern:
[FEN "rnbqkb1r/ppp2ppp/4pn2/3p4/2P5/1P3N2/PB1PPPPP/RN1QKB1R w KQkq - 1 4"]
2. d4 doesn't contribute to these plans and it's definitely not urgent, so why not try something else?