The reason it's possible with a third party application is the same reason people can play custom stuff in Beat Saber or mod other games: because players in the community devote a ton of time to modding and enhancing existing games. This has always been the way.
Meta has cut its workforce and seems mainly interested in Horizon Worlds' games/activities and glasses these days. I don't see them devoting time and manpower to develop and provide support for an app to improve the visuals, etc. for independent developers' games, especially since each game requires its own custom profile for optimal look/feel.