But you don't have assigned seats on German high-speed trains. It certainly happens (although rarely) that trains are so full that some people need to stand
I’d say that having to buy a ticket with an assigned seat in high-speed trains in Spain has been always been that way.
However, short-route trains (Cercanías) don’t have assigned seat, but they are not high-speed trains.
It will be to keep the quality of the service high, reduce boarding times, or something like that.