Why the low draft grades?

Our Super Bowl window is now closed. Our over-under win number is Las Vegas is at best nine. We are essentially a 9-8 team. Yes, we will be competitive. Yes, we could be in the playoff race. Yes, if we stay healthy and get breaks, we might even play in a playoff game. But we are not close to being a legitimate Super Bowl contender.
I am just curious, what is the difference between a perennial playoff team (4 straight years, would have been 5 last year if not for everything this team went through, yet still made it all the way to the last play of another game in the last week of the season before being eliminated) and a legitimate Super Bowl contender? One would think a team that just keeps reloading and making the playoffs every year would be in that echelon, but I’m open to being educated.