Why are train tickets called "hard seats"?

Most of the seats on the train are hard seats, so most of the train tickets are named hard seats, which is also related to its price. Hard seats are relatively cheap, so it is impossible for a hard seat on a train to be comfortable. I can only say that I try to make myself uncomfortable. We are uncomfortable sitting, staying up late and having trouble going to the toilet. Everyone in the next seat is asleep, so you can't go out and you are embarrassed to disturb others. It can be said that it is uncomfortable. Drink less water and eat less at night.