Like everything, it starts with an organic foundation then slowly gets absorbed by capitalism. They're now brand reps more than anything. Subscribe to curiositystream, BlueApron, and watch your favorite streamer take an extended, nonchalant sip from their favorite [brand] soda, replete with logo mugging the camera lens.
Big Money Salvia gets a pass because he actually puts some effort into his ads, to the point they're entertaining enough you can ignore the product placement.