Why do people think the Go/Windows is somehow inferior to the Steam Deck?

I think it's the opposite. You don't have to worry about Proton compatibility problems, and Handheld Companion is often better than the SteamOS tools. For example the default gyro mapping in HC almost always just works, while I always had to tweak it per game in SteamOS. Also Geforce Now works properly in 1600p. It really is amazing to see contemporary AAA games on very high settings and native res on the Legion Go with GFN. On SteamOS you are limited to 1080p60 and a relatively low bitrate because of codec shenanigans.