Booking a European night train confuses American travelers because the price is attached to a berth and to the privacy of the compartment, not to the journey.
The compartment is the unit of inventory
A sleeping car is divided into compartments holding one, two or three beds, and the operator sells either individual berths or the whole compartment.
A solo traveler buying one berth may share with strangers of the same gender. Buying the compartment for sole use costs more and removes that.
This is why the same physical space produces several different prices depending on how many berths are sold within it.
Couchettes and sleepers are different products
A couchette is a padded bunk in a compartment of four or six, used with bedding provided but without private facilities and without a full made bed.
A sleeper compartment has proper beds, a washbasin in many cases and sometimes a private shower and toilet, with an attendant serving the car.
The fare gap between them is large, and the accommodation type is the main determinant of the price rather than the distance traveled.
Seats survive on many night trains
Most night services also carry ordinary seated cars, sold at a fraction of a berth price, which is how the cheapest overnight fares are advertised.
A seat on an overnight run is a genuine option for short nights but is not comparable to a bed for a long one.
The presence of a cheap seated fare in a search result is the usual reason a night train appears less expensive than it turns out to be.
Capacity is small and fills early
A sleeping car holds far fewer people than a seated car of the same length, so berth inventory on any given night is limited.
Popular routes in peak season sell out months ahead, and released inventory does not usually replenish once gone.
Because the equipment is specialized and expensive, operators cannot add a car to a sold-out train the way an airline adds a larger aircraft.
How to compare it against a flight
The honest comparison counts the hotel night the train replaces, plus the airport transfers at both ends and the time those consume.
Night trains arrive in city centers early in the morning, which is usable time, though luggage storage until check-in is a practical problem to solve in advance.
They also fail differently from flights. A delayed night train usually still arrives; a canceled one leaves passengers with a night to rebook rather than a connection to rebook.