NFL players are elite athletes. They need all their systems running at peak to maintain their level of strength, speed, etc.
Imagine DK Metcalf comes down with Covid. He recovers, but has suffered some permanent lung damage. He can't train the way he used to or run as fast as he used to and when he does run he gets winded easily and can't sustain speed. He'd be broken - not the player he used to be.
Is that good for Metcalf? The Seahawks? The NFL?
It would be great for the lawyers who work on the settlement with the NFL, because I imagine it would be around $100 million.
So no, they won't be looking at Covid in terms of death, they will be looking at it in terms of potential liability, and erring on the side of caution.
I should also mention that one of the common effects of Covid is blood clotting, which could cause a stroke, particularly when combined with long-haul flights which are common for NFL players.