Isn't AirPlay pretty much analogous? Only available on a single brand of devices with an effectively DRM protected closed protocol?
Meanwhile, Google Cast is a media-streaming negotiation, handoff and control protocol marketed as a wireless streaming solution. It's not so much about packets on the wire as control commands and metadata. That, I feel, is a significant difference.
So platform support is more widespread, though your main point is valid insofar as Google determines which platforms it works on.