The point is: people who are willing to pay for games would probably never consider paying for a downloadable HTML5 game.
You might want to do some research there. People even give money to non downloadable html5 games. Reality is the opposite of your claim. The downloadable version of a web game is actually an incentive to buy the game. That the downloadable version would just be an html file, is not a deterrent. Maybe for compatibility reasons, so some devs just pack a browser, to ensure the game will actually run offline.