Because single players games are more accommodating to network issues, skipped frames, adjustments in resolution etc. You will notice an input latency spike if you're playing a multiplayer game that requires precise and quick input.

I've played around with Stadia a bit, even on "good" internet in the US, 20ms ping, 250mbps, Stadia was only really enjoyable for single players games. Even playing a relatively slow multiplayer game like Dota 2 would have been frustrating fairly often.