You'd have to run several versions of several db engines side by side, which is not even doable easily in most distros. Not to mention some apps need special niche versions, Immich needs a version of Postgres with pg-vectors installed. Also they don't tell you how they provision them — and usually I don't care because that's the whole point of using a docker stack so I don't have to.
Last but not least there's no reason to not run databases in docker.