Because the barrier to entry for this specific product (smartphone) is incredibly high. Kind of like, if you want to buy a passenger plane, you have to go to Boeing or Airbus.
If I had an airline as a client and I wanted to change the upholstery on a plane, I don’t think I’d need to have the changes approved by Boeing/Airbus in addition to having my personal data (home address, phone, and ID) sold and used to target me with advertising.