I can easily install launchers available in the market like Nova launcher and others but not the launchers made by many OEMs (e.g. TouchWiz, Optimus UI, Xperia UI, Sense UI, Emotion UI by Huawei, MIUI by Xiaomi, Zen UI by Asus, Color OS by Oppo and the Cyanogen).
Anyone having a device made by those OEMs should be able to upload the APK of their launcher, so why it is not possible to install them? As an example, see How to download and install Samsung TouchWiz launcher any Android device?
A good detailed explanation would be appreciated.