It probably has to do with the price. I've done some checking because I have wondered what the 'free covid tests' were really costing the goverment. I found one study that shows the largest percentage in their study (30%) ranged in the $100-$149 category. So lets assume between players, coaches and staff, an NFL team needs to test 100 personnel (which I think is probably close, but low-ish).
$149 per test * 100 people = $14,900 per day
$14,900 * 7 days = $104,300 per week
$104,900 * 18 weeks = $1,877,400 per regular season
$1,877,400 * 32 teams = $60,076,800 per regular season league wide
And I'm sure these costs don't scratch the surface of everything else that goes into testing, same day results, supplies, handling, reporting, doctors, etc...