The kernel should be rolling-released in the next LTS and Ubuntu have plans to go full-rolling-release for a long time, so they are working on it. But it can't be simply done. Even the best rolling release these days is much less user friendly than the most unstable Ubuntu release. For the time being we have PPAs.
Well the core can be frozen and the optional be rolling.
Example, everything pre-installed and on the ISO image is frozen, i.e. the kernel, GNOME, X.org, LibreOffice, GCC, Python, etc.
Then stuff not on the ISO image will be rolling, such as GIMP, Blender, VLC, Audacity, etc.