Absolutely nothing (AIUI) to stop you standing outside the doors of Stansted Airport and booking a Ryanair flight on your phone 2 hours before it is due to leave (it just might cost a bit, and subject to having a spare seat)
The pricing model is a slightly different issue. DB for example do not operate compulsory reservations but equally have no discounted walk up fares other than by way of the BahnCard. Whereas there are railways where compulsory reservation applies but everyone pays the same, some of the Asian ones like State Railway of Thailand and KTMB (Malaysia) spring to mind.
FWIW I’ve (during disruption) booked a flight very close to departure before. I recall sitting at the outside benches at Euston booking one from Heathrow on another airline to replace one I had from City which was cancelled. I’ve also realised I was going to miss a flight while on a delayed train before, got my phone out and changed it to a later one. Many people think the concept means booking the day before, it doesn’t, that is mainly a pricing model issue.
One upside of it is to get rid of the Euston scrum, as why rush to the train if you know you have already reserved your preferred seat?