You make an interesting observation. But from the airlines’ viewpoint, then isn’t the need for a reservation a method in which they can achieve higher load factors and yield manage the number of seats on the plane.
In the old days there were few price options per flight, capacity was strictly controlled (do you remember the days when BA/AF were allowed to share revenue and capacity between LHR and Paris ?) and there was no such thing as yield management.
In fact, the only true air service where everyone travelled with open tickets (and with a seat guarantee) was the famous BEA Shuttle from the mid-1970s which plied between LHR and BFS/EDI/GLA/MAN. (On BA Super Shuttle, which appeared years later, reservations were required for some of the cheaper promo fares).
By the way, although UK trains don’t need check-in the system can be different in mainland Europe.
In France, reservations are compulsory for SNCF’s TGV services and also for Eurostar (which is essentially an SNCF product).
Indeed Eurostar has a small number of “tip down” seats in the vestibules. These are offered to those passengers who buy tickets on departure and when the train’s normal seating is fully booked.