your snaps get executed inside the core snap which is an ubuntu “rootfs”, to keep libc, linker et all compatible, snapcraft uses indeed the ubuntu archive for build and runtime packages when assembling a snap. even on other distros it would have to do that in the current setup. a build on windows or macos would have to use a minimal ubuntu container to build …
there is an approach to overcome this (at least for other linux distros) so you can also use fedora or suse rpm archives etc, discussed at Introducing base snaps