It’s because they’ve intentionally made it possible to cheat their system and get away with steroid use, and the trainers and officials are the ones helping the athletes.
The list of 100s of athletes failing tests paints a different picture. USADA performs random testing at random times. Cheaters are going to cheat but the US is not complicit in that cheating. It’s a constant arms race cheaters find ways around testing and testers find ways to detect the new methods.