Wouldn’t it be easier and cheaper for them to just put stock android and add the bloat apps on top?
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In order to make their phones stand out or add features that aren’t available in the base android experience.
Or to make the UI look more like an iPhone in the case of a lot of Chinese manufacturers.
Branding: in a market where there are multiple manufacturers selling what’s largely the same internal hardware, it’s the extra customizations each manufacturer chooses that gives a reason to pick a manufacturer.