Just doing some quick and dirty analysis, I worked out the average number of ranks gained by being (non) terminal with this method, and got the following numbers:
Non terminals gained ~4.3 ranks
Terminals lost ~2.8 ranks
So that one simple fact accounts for quite a bit. Like I said, this analysis was very quick and I make a lot of simplifications (I ignored all victory and treasure cards, as well as some, uh, variably terminal cards like Throne Room or Tribute, without removing the ranks they affected, and I'm using all the cards (except potion costs) when there's different numbers of cards of each cost, which messes things around) - but I think the point it makes is clear. Or perhaps it isn't. You could argue that people, on the whole, undervalue non-terminals... but I'd say it's just that non-terminals are easier to fit into more decks, and can be bought in larger quantities more easily.