I wonder if it's a consequence of Valve's core business is dealing with delivering data for games regularly in the 10's of GB range, they assume most of their customers are on connections good enough for game downloads so they must be using the store and chat via the same. I guess if it's not unpleasant enough for a large group of people to prevent commerce from happening, then it's not worth putting engineering effort into (or however they allocate effort at Valve now).
Also Valve has relatively small workforce. That being only couple hundred. With good part working on two of the most popular games. And then other projects. In the end they don't have that much idle hands.
No, it's Valve just not giving a hoot.
People don't seem to (want to) remember, but before the run-up to the Steam Deck, Valve was really coasting.
Big Picture was riddled with bugs. Controller configuration was riddled with bugs. The Steam in-game overlay was terrible, and had been terrible for years. Their internal browser was a custom stripped down 5+ years old Electron or Chromium Embedded Framework monstrosity, begging to be exploited somehow. Chat and voice chat was terrible, didn't scale properly and was missing many of the features of things like Discord.
And things had been stagnant like that for 7 years, if not more. Their built-in browser and chat functions are still terrible. In regards to chat and voice, I hope they cave and add Discord integration, so we finally have one service for PC, Xbox and Playstation.
Their store was and is still all webviews, the pages themselves are terribly optimized and many of them aren't even controllable on the Deck without touch. You read that right, the Big Picture cursor doesn't work on some store pages, which renders them unusable on handhelds without a touchscreen / hardware (emulated) mouse.
Steam is miles ahead of the Epic or GoG UX, but it still has lots of warts.
Steam Input controller bind customization is off-the-map nuts with how broad it is now though, major props for that. And the 'new' Big Picture is a vast improvement.