In the book night, why are people unable to sit down?

There is no room to sit.

Packed into cattle cars, the Jews are tormented by nearly unbearable conditions. There is almost no air to breathe, the heat is intense, there is no room to sit, and everyone is hungry and thirsty. In their fear, the Jews begin to lose their sense of public decorum. Some men and women begin to flirt openly on the train as though they were alone, while others pretend not to notice. After days of travel in these inhuman conditions, the train arrives at the Czechoslovakian border, and the Jews realize that they are not simply being relocated. A German officer takes official charge of the train, threatening to shoot any Jew who refuses to yield his or her valuables and to exterminate everybody in the car if anybody escapes. The doors to the car are nailed shut, further preventing escape.