2. Steelers starters played more in the preseason
Preseason is just that: it's preseason. We can't read too deeply into the results or gloat over the fact that Kenny Pickett had a perfect passer rating. These are exhibition games that are by no means guaranteed to reflect what we see in the regular season.
While the statistics from the preseason may not be relevant, playing time among starters may prove to make a difference. Mike Tomlin wasn't about to let his players be ill-prepared for a daunting Week 1 matchup against the San Francisco 49ers. As such, almost all of his starters on both sides of the football played multiple series in each of their three preseason games this year.
The 49ers, however, are not big on playing starters during the preseason, as Shanahan would rather limit the risk of his starters sustaining injuries ahead of meaningful NFL football. There is risk and reward on both sides, but the added snaps during the preseason could favor the Steelers.
In addition to the starters already being warmed up from their preseason reps, the Steelers also practice tough and had live tackling sessions during training camp that many teams are reluctant to do. Mike Tomlin likes to start the season out right, and these added reps from the starters could give them an advantage over the 49ers this week.