Snaps are built on a "base snap" which in turn is derived from Ubuntu LTS. Further, they can bundle debs from the Ubuntu repositories using the "build-packages" and "stage-packages" manifest keywrods. One bonus of this design is that the people writing the manifest don't have to worry about how the deb dependencies themselves are built. They only need to worry about building any third-party dependencies.

Now, flathub does maintain a collection of "modules" -- basically prewritten manifests for building various common dependencies from source. But that collection is still quite small, and writing a working flatpak manifest can involve duplicating a lot of the hard work of Debian and Ubuntu package maintainers.

Recently, Fedora started something promising from a maintainer's point of view by:

* creating a flatpak runtime based on fedora releases, and

* allowing one to specify existing Fedora packages as dependencies instead of having to figure how how to build all the dependencies from source (https://docs.fedoraproject.org/en-US/flatpak/tutorial/).

But it is still in its early stages.