I thought the real reason manufacturers do this is because they're in a very low margin industry. For computers they get some deal with software companies to pre-install their product, and similar for cell phones with service providers.
With that said, differentiating by custom UIs on top of the base platform (i.e. the Android approach) would be much better, and would be actually-useful differentiation rather than a matter of "well which company's going to fuck me over the least with their preinstalled shitware?".
This also isn't exactly universally true. HTC was pretty responsive in getting Lollipop out to my One M8, for example, even with its whole "Sense UI" shebang.
It's still only one sort of business strategy. Not every hardware company is necessarily pursuing this line.