Google isn't a name that people associate with gaming or gamers, buit they do have a name associated with dropping products after a year or two. I wouldn't trust Google to still be supporting "Stadia" in 5 years time, whereas I do know Steam will still be there.
Also, Google's reputation for "do no evil" has been rapidly eroded in recent times and people are more and more wary about giving this company any more personal data than they already do. Contrast this with Steam, a private company that gamers trust with their data.
I think Google is once again reaching here into a space too late and without anything really compelling to make people switch away from existing Xbox/Windows/Steam ecosystems.
Edit: One other additional aspect to this: it doesn't really matter how fast Google's servers can stream the data, most people's ISPs do not have great consistent connections. Hell, I have Google Fibre at home and occasionally I get issues streaming 1080p Netflix (stuttering etc). Most people have services with bandwidth caps and do not get consistent enough speeds to support streaming games. At my parent's house in the UK, the situation is even worse with lower caps and very often there are disruptions in the connection and so the Netflix stream will turn into a blocky mess for 10 seconds or so.