2nd Team Getting Their First Start
Most of the players on the O and D-lines are holdovers from last year. It's not a talent issue. IMO, I think it's an injury or conditioning issue (or both, maybe).
I would posit a potential third possibility: hangover. Whether Super Bowl, new big contract, familial, or other factors there are a lot of external pressures on this team this year.
From Nink actually pulling the plug to Branch doing the same except sticking around to Solder looking for all the world like he wants to be anywhere else but the football field there is evidence that football isn't the priority for some very key positional players. Not saying this IS the truth, just that it's another potential possibility.
It seems to be the new normal for pro sports (especially basketball in the NBA): the "I don't give a crap" regular season. The difference - there are only 16 short games in the NFL.