I'm not a sales expert or anything so this is just my observation. 

There are many factors and one is that people loves buying more in their own terms rather than  having a seller present a product to you. 

The reason could be first is trust, that's where advertisements come in. People trust products that are advertised in TVs or branded by famous artists, that's human nature. If an unknown salesman would sell a product to you. there's no trust involved and you would feel like that the salesman is just ripping you off.