I thought Google was just search and a few services like Drive, Docs, Gmail etc Shouldn't Stadia fall under the branch of Alphabet and omit all instances of using 'Google' in their marketing? Why use the Google name for something not remotely related to search?
Because the Google brand is still orders of magnitude more recognized than Alphabet especially in mass market. I would wager a decent chunk of their target audience doesn't even know what Alphabet is
Google is much more than that; think Google Cloud, which I think this falls into. Also even hardware (Chromecast, Pixel) falls under the Google umbrella!
Brand identity, I suspect. They're the only folks in the world who are allowed to (legally) prepend "Google" to a service or product, which is essentially free marketing.