The Android OS is created and developed by Google, so they have the first rights to put anything they want on their software before other companies such as Samsung, HTC, ATT, and so on add their own apps. Most companies don't attempt to remove Google's software because
A) Most of the apps made by Google are useful like the Google Search app or Google Chrome and
B) Most companies either use the open source code, where it would be a dick move to remove their apps since they are letting you use the OS for free or have a contract with Google for the OS, in which they are not allowed to remove Google's apps in their own version of Android.