The alternatives are all closed-source protocols. A ton of things are backed by Google. It doesn't mean that they have anything to do with it other than them being consulted in its design and providing some official support. And considering Google makes the most popular mobile OS, and also the most popular TV OS (which is relevant considering it's smart home), it's good that they're in on it. Otherwise you end up with a fractured situation like you currently have with things like Airplay, Chromecast and Miracast, which are all the same thing with slightly different implementations because they couldn't agree on a protocol.

And I'd vastly prefer it that Google (and Apple, and Amazon, and Home Assistant, and IKEA, and Philips, and...) all agree on the same protocol than each vendor making up its own thing.