Why is consuming drugs even just once, dangerous for driving on public roads?

You answered correctly. Consuming drugs, even once, is dangerous for driving because it can make you unfit to drive; even a single dose may lead to impairment that lasts several hours, affecting your ability to react and make decisions. Drugs can also cause misperceptions or hallucinations, increasing the risk of accidents. German traffic law is very strict about drug use: any impairment makes driving illegal and dangerous for everyone on the road. Always make sure you are completely sober before driving.