Most trains in the US do not have seat reservations either. A ticket is for a route, and permits you to travel on a train. You do not “book a seat”, you pay for transportation. A 1st class ticket permits you to be present in the parts of the train marked 1st class.
However, the Swiss Railways run enough trains, and run long enough trains, so it is very rare for there not to be enough seats. Average occupancy is around 25%. I travel 1st class a lot, and have never not had a seat.