That's more because GNOME has completely dropped GTK2 while other projects continue to use it. In the past that wasn't true, GNOME 2 apps depended on a separate library called libgnomeui that had widgets specific to GNOME 2. All the actively-developed GNOME apps ported to GTK3 long ago and so they dropped that dependency which is why you don't see it anymore. The only apps left using GTK2 are non-GNOME apps and so they never depended on that in the first place.