You can indeed install everything manually (it won't be just lutris though, there are many possible tweaks).

SteamOS/Bazzite/uBlue project are Linux distros with immutable core. The immutable part is tweaked, tested, fixed in time and can be easily switched/rolled back. Many people (me included) find it much nicer experience to get some battery included base that someone more competent put together and that can't be easily broken. So instead of figuring out how to put linux parts together you find immutable distro that fits most of what you need and then install just what is missing.