McDonalds is a fast food business, they are there to make money. Their target audience are the average Joe and Jane on the street who are looking for fast, cheap and reasonably decent tasty food. These people don't tend to be concerned about how healthy the product is.
Games companies are there to make a profit. That is what keeps them going. Most of the income is from the average Joe and Jane on the street who want something enjoyable to play and also something that they recognise (i.e. brand). These people are unlikely to care about what the finer ethical concerns or how sophisticated the design of the game is if it doesn't fit into their definition of what 'fun' is. They are usually more taken by the experiences that happen continuously such as the free running in Assassin's Creed which is simple to do by the player but looks frigging cool in game.
A good game from a design point of view is not always a good game from a 'average Joe's' point of view (e.g. Ico). Until they change their mind, that is what the industry will keep giving them because that is where the money is.
Aside: The note on Assassin's Creed on killing guards next to each other without reaction sounds more of a bug rather then a design decision.