The USDA has not made it illegal or unlawful to cook steak rare. They recommend against it. A lot of people like rare steak, and will pay a restaurant to prepare a rare steak. So they do.
Do you mean you grew coltures of bacteria from various levels of cooking and compared? Was this work peer reviewed?
Order of magnitude, ~15% of beef consumed is quality steak cuts. US residents on average eats 67 pounds of beef (so 10 pounds of steak) per year. If eating steak cooked less than medium-well was dangerous in a practical/meaningful sense, it would be common knowledge (based on lived experience) that steak should be catastrophically over-cooked as a food safety measure.
They exist to be an ultimate authority, and that constrains them from presenting a compromised position, even if that compromise is something people find widely acceptable. The USDA simply isn't in a position to guarantee enough safety in this case to balance against their liability.
Safety is compromise. Eating rare steak is risky, but most of the risk is mitigated. Getting sick from eating rare steak is, itself, rare.
The USA department of agriculture begs to differ. https://ask.usda.gov/s/article/Is-a-rare-steak-safe-to-eat
Personally I never place much trust in random comments on the internet.