This is easy. It's because it's too easy to make any sweeping conclusions about why Kyle Shanahan has failed you, or why Jimmy Garoppolo stinks, or why George Kittle is brittle or any of the other snappy reasons they lost to the Arizona Cardinals and are 0-1 like some common Dolphins or Browns or Bengals.
The 49ers weren't as sharp as you remember them, true. They did have moments, like the Raheem Mostert touchdown, but for the most part they were slightly off, and in one significant area (DeAndre Hopkins) they were spectacularly outmanned. It's all reparable, but for a week punishments must be meted out because... because that's the nature of football between games. Overanalysis unchained, turbo-whining and unfocused rage are the coin of the realm, and the 49ers' preseason narrative flies directly in the face of losing at home to the Cardinals.
And that may be the problem. Because the nearly unanimous sentiment was that the 49ers are uniquely positioned to defy the Super Bowl losers fade into the background law, so being stopped at the goal line and getting Kittle hurt on top of Deebo Samuel and Brandon Aiyuk and being outplayed by a team that was 5-10-1 a year ago seems like proof that they actually aren't good enough to violate that law.
In fact, the Cardinals look better than they've been since perhaps their Super Bowl year (2007), and the 49ers don't have enough weapons to cope. Depending on the Kittle prognosis, this could become a recurring grousefest, but because its only the day after First Sunday, nobody really knows much of anything. We'd need three weeks of this before we could make any kind of conclusion, and since the next two games are in New York against the preposterous Jets and Giants, the next two conclusions you are likely to jump to are that the 49ers are fixed, and not in the veterinary way.
In a way, Sunday's outcome was a good one because it makes every week that much more chewable. Blowing out the Cardinals would have jacked up the fan base's well-known smug level, and it's better that they have to work for this season more than they did last year. After all, this is entertainment and spoiler alerts in Week 1 aren't good for anyone. This should be more uncertain and more erratic and anti-predictive. The fun of not knowing is always better than the predictability of knowing, and last year's early winning streak took some of the mystery away.
Now, some of that mystery is back, and until Sunday morning it will remain. Lean into it. You'll learn to like it. It is, after all, the one thing Shanahan hasn't tried yet, battling week to week and fighting to make the postseason. Embrace the new, at least while it lasts. Which is probably until their next real test against the Belichicks in Week 7.
See? We've already got you programmed into Hallowe'en. You're entirely welcome.