Apps and Interface looks different Why is Android on mobiles made by different companies different, and why do apps like Facebook also differ likewise?
The operating system on your phone isn't put there by Google: it's put there by the manufacturer of the phone (the OEM). Google gives OEMs the source code to Android, and the OEM has to integrate its drivers into the phone. The OEM can also make any changes to Android, as long as it still passes the compatibility tests (so that apps will run on it).

The OEMs like to use this facility to make changes, first to add their own branding, and second to add features they regard as "value add". OEMs want to differentiate their phones from their competitors this way. They don't want to become like PC manufacturers, who don't have as much ability to customize, because PC manufacturing has much lower margins.