And the 75$ upfront cost does exist, you need a controller. You could get a controller for cheaper, but it wouldn't be as easy to use or be usable with the chromecast ultra, which seems to be where the best experience is. Nonetheless you still need a controller and most people wouldn't settle for something that is terrible to use, so the cost is still a consideration.
Especially in the 'casual/occasional' gamer market, where they don't care enough about the difference between 1080p and 4K. Most are probably just interested in being able to play the latest hot game. That's exactly the same market Microsoft is targeting with the Xbox Series S.
Last time I signed up the Stadia Pro upsell was mandatory (you had to provide a CC). This was annoying because I was interested just in Destiny 2, which is free.
I can go on the store and buy Cyberpunk 2077 and start playing on my phone if I wanted to.