i guess megameters weren't needed that often in the past and they never caught on later.

Probably too easy to confuse with mm.

Interestingly megaton is rather common unit but not teragram.

Only place I've occasionally run into metric prefixes above km is in astronomy within proximity to our planet. You just don't need them for most terrestrial applications, and as soon as you get out of the solar system people switch to parsecs and light years.

I suppose people also have a basic sense of how long a km is, where that goes out the window with anything bigger. Especially anyone who has gone through the military, has an intuitive feel for how distant a "klick" is.