Lots of QB injuries don't help. No Rodgers, Burrow, Watson, Cousins, Jones, Tannehill, Jimmy G, Murray (for most of year). Stafford has been banged up. Plus there are several rookies and 2nd year guys playing, many of which were not exactly high draft picks. And 2 of the franchise guys i.e. Lamar and Hurts aren't exactly known as prolific passers or play in heavy pass offenses. Kelce has been hurt and the Chiefs WR's are bad affecting Mahomes numbers.
Teams that have remained healthy have very strong numbers like Tua, Dak, Goff, Allen (virtually identical to last year), Lawrence is up some, etc.