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.
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.