Maybe it's because most reviewer are jaded because it's industry standard for gaming companies to promise the world and deliver on only a small part of that.
In general when a gaming company says "and we're going to do..." that's generally qualified with "if there's enough user adoption and demand".
To believe in Stadia, you have to trust Google will deliver on promised features, trust that Google will convince publishers to develop for it's platform, and trust your internet connection. Google really only has control over one of those things.
Also, I wouldn't at all be surprised if Comcast, Verizon, or AT&T are holding a grudge after Google Fiber and might take steps to degrade the Stadia experience as a little fk you to Google.