Why is e4 e5 nf3 nc6 d4 cxd4 nxd4 nxd4 an innacuracy?

3.......Nxd4 is a big mistake.     why ?   in wide open positions  ( as this is, when white pushed 3 d4 and Black too it ) tempi matter a lot.      Black is giving up a well centralized knight to get White's Queen to a centralized position.     Black has no pieces in play and White will continue his vibrant development -- soon to take over more key squares

i actually meant e4 c5

#3
It is not that bad, but it centralises the white queen
https://www.chessgames.com/perl/chessgame?gid=1896808 

After 1 e4 e5 2 Nf3 Nc6 3 d4 exd4 4 Nxd4 Nxd4 is playable too
https://www.chessgames.com/perl/chessgame?gid=1583023 

4.. Nxd4 looses a tempo. Before that move black and white always had as many pieces developped when black had moved but after the capture white preserves the extra tempo even after black's move.

1. e4 c5 2. Nf3 Nc6 3. d4 cxd4 4. Nxd4 Nxd4? 5. Qxd4 is inaccurate for exactly the same reasons.