I'd argue that's only because km is a very ingrained part of everyday parlance. The use of a megalitre for example would not raise an eyebrow where I live.

Similarly megawatt and gigawatt are pretty common and for things on a global scale terawatts is not unheard of, I don't see why it would be so different for TWh to resort to an arcane way of formatting it.

Don't get me wrong I'm not autistic enough to not see your point, it's just tilting to me to make these extra jumps. Esp. when Wh is already a cursed unit