Dak historically is not a hot starter. In his career he has only topped 300 yards passing twice Week 1, 2019 and 2021.
Like other have pointed out, he and the offense never played in real game conditions during the preseason. Not only was Dak not the sharpest, but the receivers dropped their fair share of very catch-able balls.
Hell, last year he started off worse with a stat line of: 13/24 for 143 yards with 0 TDs and 0 Ints
So if you were expecting 300+ yards and 3+ TDs, week 1, then you might not see it for a long time. Especially if Dallas always sits key offensive starters all of preseason.
JMHO
In any case, he was easily the best QB on the field, so there is that.