Airlines need to be regulated and treated like public utility which is exactly what they are.
When you fly, you usually have a choice between lots of airlines. So there's nothing "public utility" about it whatsoever.
Airports, on the other hand, are considered public infrastructure. There are also sometimes routes that are only served by one airline, which are sometimes regulated accordingly. But that isn't the general case, nor should it be.
Public utilities include things like electricity, power, gas, sometimes telecoms. Being a monopoly is an inherent part of it.