According to an interesting topic here, about 5/6 of the retail price goes to the publisher/developers. Factor in packaging, shipping, and the most you could ever reasonably expect a retail non-indie game to sell for digitally is 25% off. So, $30 rather than $40.
But that can't happen, because they can't undercut their #1 source of income. Digital sales - even discounted - would only make up a small portion of total sales, but it would anger the stores who sell their games, and they may lose their support. They can't risk that. The market isn't anywhere near ready to go all digital.