Competition dynamics for air traffic are different. Railway infrastructure can only support a very limited number of competing companies before you run out of capacity, and especially on classic mixed traffic routes it is very easy to run out of capacity. Once that happens, rail operators have to start competing for paths instead of directly for passenger, which can definitely lead to misalignment of incentives.
On top of that, there's the problem that it's physically impossible to run competing services at exactly the same time. This means that for customers with schedule constraints (i.e. you need to arrive somewhere specific by a certain time at latest), competition becomes much less effective, because you're no longer able to freely choose among the competing operators, but are instead forced to simply take the train that arrives at the right time.
A similar thing is for journeys that involve changes of trains – it's physically impossible to have attractive connections (i.e. without hanging around the station for ages) between more than one or at most (if that) two operators per route, because trains running along the same line always have to be separated by at least two or three minutes.
With air traffic it's different. While airport and air space capacity isn't quite unlimited, it's still not as limited in the way a mixed traffic railway is. Plus a much higher proportion of air traffic is holiday traffic and other long-distance journeys where even a few flights per day would be considered frequent service, so competition is much less limited by that fact.
On top of that, there's the problem that it's physically impossible to run competing services at exactly the same time. This means that for customers with schedule constraints (i.e. you need to arrive somewhere specific by a certain time at latest), competition becomes much less effective, because you're no longer able to freely choose among the competing operators, but are instead forced to simply take the train that arrives at the right time.
A similar thing is for journeys that involve changes of trains – it's physically impossible to have attractive connections (i.e. without hanging around the station for ages) between more than one or at most (if that) two operators per route, because trains running along the same line always have to be separated by at least two or three minutes.
With air traffic it's different. While airport and air space capacity isn't quite unlimited, it's still not as limited in the way a mixed traffic railway is. Plus a much higher proportion of air traffic is holiday traffic and other long-distance journeys where even a few flights per day would be considered frequent service, so competition is much less limited by that fact.