Who I have the same questioned this topic

You must be very ignorant when it comes to Android, or AOSP, which stands for Android OPEN SOURCE Project. The entire OS is completely open for anyone to see, modify, and use, that's why you'll find custom ROMs on different phones. Phone manufacturers will get the stock ROM from Google, and then tweak it so it will work with the specific phone's hardware, remove features the phone can't support or they don't want on it, apply their own launcher (like Touchwiz), and add any new bloatware. When they are done, they give it to the carriers, who tweak it how they want, be it to make sure it will work on their networks, removing any features they don't want usable, adding bloatware, etc, and then they pass it on to the consumer.