The NFL (and other major sports organizations) have no incentive to catch their athletes beyond a token amount to make the general public think the league cares about it. They don't want to catch too many as that could lead to a PED scandal that damages the reputation of the league; for example the BALCO scandal in the early 2000's MLB. Plus PEDs allow their athletes to stay healthy and perform at higher levels.

There's a reason athletes refer to PED tests as IQ tests. Only the very dumb or careless get caught, but the reality is nearly all of the athletes in these leagues have used PEDs at some point in their career.