Re: Reinventing it all
Because [the phone makers] need to differentiate themselves from the rest. If all Android phones ran generic Android with the same software features, people would base their choices on price and hardware specs, and the Chinese companies would have 100% Android market share.
Just about 100% of all phones are made in China anyway, whatever the name on the box.
Moto (Lenovo) choose to apply minimal customization, that option is available, while other manufacturers add their own customization (and still get significant market share, so someone must like -- or at least not vehemently dislike -- what they're doing). In some cases the OEM changes are related to specific hardware (Samsung's S-Pen, for example) and couldn't feasibly be removed without impacting on functionality; in others it's just an OEM-specific theme, and one really wishes that it could be removed.
There are two parts to the problem, though. One is the OEM-flavoured skinning of the OS itself, which is undesirable because it means that updates will be provided more slowly -- if at all -- than 'vanilla' Android devices. That's a problem that really needs to be solved.
The other part of the problem is the provision of OEM-specific applications (such as Bixby). These applications are typically provided for the OEM's benefit -- to skim the user's data, to display advertising, or to provide vendor lock-in by storing the user's data in proprietary formats. It's understandable that the OEM doesn't want to make these apps removable, and equally understandable that the savvy user's response is to root the device.