This is one thing that Telegram did right with their .tgs animated sticker format (based on Lottie) and that somehow no one else in the message space seems to want to take the effort to replicate.
JSON and vector-based, heavily optimized for filesize and battery use.
Vector-based limits or at least requires rethinking the artwork design, and for stickers that are mostly ripped from game assets that might be effort than it's worth for Steam stickers.
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).