Every single one of those websites use HTML, a common media type, with semantics defined in that standard. What's missing are the application-level semantics like what's a "flight search" and "flight booking", which can be solved with linked data. In fact, this already exists! [0]
In theory, using a common media type and linked data vocabulary, one can make this hypothetical scenario of an automated machine-to-machine flight booking system happen. In practice, it requires either changing how people think, or build APIs to begin with. It's a steep uphill battle to change how people think, making this happen is much easier.
In theory, using a common media type and linked data vocabulary, one can make this hypothetical scenario of an automated machine-to-machine flight booking system happen. In practice, it requires either changing how people think, or build APIs to begin with. It's a steep uphill battle to change how people think, making this happen is much easier.
[0]: https://schema.org/FlightReservation