This is because the developer community, aside from some long-time system hackers like Linus who know what's at stake (most of whom are Linus's age or older), do not think in terms of building software that will last and adapt to changes. They think in terms of "old and busted" and "new hotness" and that their job is to migrate everyone off the old and busted and force the community to adopt new hotness. Hence the churn in GNOME, hence systemd.