They should be but I don't think they are tested on any kind of regular or consistent basis, or they are not tested enough. I think the authorities are too scared because i'd be surprised if many of the top players don't register for something and they simply don't want to know so they don't have to act.
There was a story a while back that Dr Ferrari, who doped up lance Armstrong, had been working with Barcelona (and I think maybe other Spanish teams) and he had frozen blood from them discovered during the investigation.
I am also aware that there are people who use drugs even at amateur level running, so the idea that most top-level sports aren't rife with it seems absurd. If people will do it for small-time glory and a few hundred quid, what will they do in a sport like football?