To be clear: the minimum word count of 20,000 for Hosted Games is total word count, not average play-through length.
You have to remember, it costs us about $1000 to review and publish a game to the different platforms. At a bare minimum, the game has to be long enough that readers will find the ads sufficiently annoying that some portion of them will pay $1 to turn them off. And it has to be in sufficient quantities so that we at least break-even on that proposition (so, > 1900 people have to pay $1 on a 20k word game in order for us to not lose money on it).
That said, we much prefer to publish games that we can charge $2 or $3 for, for example.