Games don't appear just like that on the shelves in shops. They need to be distributed, so you need distribution channels. Also you need to buy shelf space to make it noticeable, without it the game ends up somewhere in the back of the shop. They need to make and place ads. As a distributor you buy many ads and thus get a discount in magazines.
All this does require knowledge and expertise, which a developer normally not has. It's not impossible but it's hard and you don't get any money until the end, which is probably the hardest part.
