Your analogy doesn't work here. Going to a restaurant is like using an app store. Installing apks is like cooking at home. Nothing stops you from cooking a meal that will get you sick.

Now imagine that every restaurant in your city is owned by one of two megacorporations and they really don't want you to have a microwave at home, let alone a stove. They expect that you will get all your food from them. This is where it's going with apps right now.