- No e-commerce sites charge money on any app stores for users to download their apps since they want the largest audience possible shopping them.

- Google Play only takes a cut of in-app purchases of digital goods and not physical things. Most e-commerce companies would not survive if Google took a 30% cut of everything sold since margins in many industries are in single digits (electronics/computer parts for example are generally less than 5%).

1. There is little money in google play.

2. If the retailers are big enough, they can easily strike deals with carriers/ phone manufacturers. Or, like Amazon tried, create their own market place.