Say the engineers have done massive internal performance and stability improvements, better filesystem and what not; but those things are almost invisible to regular users. Most users want something more tangible, otherwise they will have hard time to justify spending few hundred bucks as they will feel "it's the same as before, I see no difference".

It's also (my theory) a bit of identity thing. All big companies slightly tweak their logo (even if it's just changing or removing shadow thickness) every decade or so that it feels fresh. The same applies to UI of applications and OSes.