The reason is when you're buying the font for a desktop, it can be used to author text on one or two machines, by one person. When you have a game the game authors those texts on many machines so the situation is the same as if you'd buy a license for each player separately.
So as long as the font has a pay per usage model a game would constitute many uses. The font makers realize your game company is not going to buy the fonts for each game separately so they cap the price for something like in this case 20 licenses, which acts as unlimited uses.
This is where it gets a bit murky. If the font is NOT being used as a font but as a bitmap then you don't necessarily have to pay the font price. As it would be the same as sending artwork to press. Unfortunately the font creator is free to use whatever licensing model he chooses so there's nothing to stop them for charging more if you use red underpants. In reality people tend to charge you what you get for it. So what you're really seeing is cost going up as the value for you goes up. This kind of price fluctuation is perfectly normal, if you go to shop in an area with richer people prices also go up.
So you're just getting a higher value for the font hence you pay more. Another way to look at it is this: If they would charge you the same as the desktop model you'd be paying $20 for each game sold, so you're actually getting a good deal here.