Oftentimes there are many ways to play that aren't absolutely horrible, it's just that people tend to follow suit with whatever is in fashion -- maybe it's safer that way. Who knows, black's plan here might be reasonable, although there might be some subtle way for white to prove an edge, although I don't care to analyze at this time.
Why do people play ...Nf6 in the ...Qxd5 Nf3 Scandanavian?
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I guess it's people not knowing the book lines - or really understanding the opening. For Black, playing O-O-O as quickly as possible is the main point of this variation, surely? So Nc6 seems essential. But for those who haven't looked at the line with 8...Nxd4 in your second diagram above, it must seem pretty scary. I haven't been playing the Scandinavian long, and no one has gone down this route as White against me yet, but I'm looking forward to the opportunity to play it.
If you explore the lines, I do think black can get equality. But I play this myself because I believe it's more comfortable for white and gives plenty of winning chances. Here is a good place to start:
https://www.youtube.com/watch?v=KMd4op5DG_g
I've been preparing the Nf3 line against the Scandanavian as white. Overall, I've been very pleased! Against the main lines white seems to get very nice positions. Here are some examples
Just one problem. If black plays the "sideline" (I can't figure out why it isn't the main line!) by omitting ...Nf6 and pressuring d4 as fast and as early as possible, I can't find any advantage at all for white!
The line seems to score well for white in the database here on chess.com, but this seems to be misleading, since black seems to be frequently playing moves that aren't best. If you follow some of the lines along, black does very well.
So, why isn't this line (with no ..Nf6) more popular, and does anyone have any recommendations for white?