All I can see is that 4.Nf3 does allow black to take the game down independent lines with 4...Bg4. What's white's best then? 5.Be2? That seems to be far and away the most popular choice.
I know a Scandi player usually expects a setup with Qa5, c6, Nf6, and Bf5, and a pretty solid position. Like a slightly askew Caro-Kann. White may be better off keeping him in that box, rather than allowing black to start calling the shots so early on...even if just psychologically.
Of course I know it cannot be a bad move, but I could argue it may be pretty good! Compared to 4 d4, white does not lose any time in developing -- continuing to augment a big part of his advantage, and I do not see how this is punished. Both 4...e5 and 4...c5 are extremely premature with opening the center (white could respond d4 right away), so what does white lose by developing very quickly with, for example, Bc4 and 0-0, while not commiting his d pawn (it will probably go to d4, but it is still good to have the option)? It seems like this would increase the chances of putting early pressure on black to hopefully make life more difficult for him. Of course it could easily transpose back into d4 lines, but this move order seems better. It is not like white ever loses the option of playing d4.