For the same reason Mitch Trubisky once came 3rd in QBR. It favours mobile QB's, especially those that can avoid a sack and turn a 6 yard loss into 4 yard gain. A bunch of negative plays with the arm can be overcome with positive plays with their legs.
No system is perfect for these things. Even in his best season Kyler was 7th overall in 2021, and every game other than the Rams I'd say has been worse than his worst regular season game that season.
Also there's been some pretty poor QB play this year. Mahomes, Rodgers, Hurts etc of all been down.
It's insane to me that Kyler is 5th and CJ Stroud is 12th, Jordan Love 14th, Jared Goff 15th, Baker Mayfield 16th and Sam Darnold 18th. All of whom have clearly been far, far superior to Kyler. Heck, Caleb Williams is 26th and I'd say he's been better.