There is one simple reason. Why pay more to make something when you can pay less and get the same results.
Confectionery and other junk food is a class example where chocolate bars, packets of crisps etc have gotten smaller over the years, but the price has gone up.
Making a giant leap in visuals will cost more and make them less money. If they can keep things basic, they need less time, less skills, less staff and people will still buy the game so profits will be higher.
Whoever said life to be like a box of chocolates clearly didn't know what he was talking about.
Life is more like a game of bumper cars. At every turn there is a possibility you will get screwed.