That's just because no one actually uses HaikuOS or RedoxOS (no offense to these wonderful projects of course). If these came even remotely close to the popularity of Linux, you'd have remixes, spin-offs, and eventually full-blown distributions.
You can try to prevent it with project structure, but even something really unified like FreeBSD ended up having derivatives like GhostBSD as well as full-blown divergent forks like DragonFly BSD that are not necessarily compatible anymore with upstream.
You can try to prevent it with project structure, but even something really unified like FreeBSD ended up having derivatives like GhostBSD as well as full-blown divergent forks like DragonFly BSD that are not necessarily compatible anymore with upstream.