Android is a gift, not a product. Android needs fixing to work properly because it doesn't work out of the box. Why? Because hardware changes from OEM to OEM, the government require mandatory support for features that aren't included, the customer (AT&T etc) require support for their apps or services, some very important ones, and last, but not least, it's buggy.
It takes about 6 months for a dedicated SCRUM team to knock a version of Android out that meets a major US carrier's requirements after Google releases their code to the community. I know because I've done it. Verizon has about 6000 requirements for their devices, Sprint and AT&T are not far behind. On top of the carrier requirements, which could be anything from implementing a custom address book sync adapter, to ensuring AGPS works accurately, you need to meet US Government requirements. CMAS is a great example. Further, there may be a stack of accessibility items that need to be done, although, Google dramatically increased their support there from ICS. So, once you've done the carrier requirements and the regulatory ones, you also then need to fix the bugs. These could be part of the open source - just look at the issues at code.google.com to see the outstanding ones, or they could be the result of the changes your chip set supplier made to have the code work with their hardware, or their proprietary codecs, etc.. Google may not have gotten around to fixing their bugs yet, but that doesn't mean you'll be able to ship a product today with without fixing them. Should those fixes be pushed back to the open source tree, absolutely, but are they? Why let a competitor benefit from our hard work? Also, you need to weed out any bugs that are unique to your hardware configuration - that could be caused by touch screen firmware, or the modem, bluetooth, or some other piece. Then, finally, you can decide if you want to reskin the UI or add your twiddly improvement. Sometimes that's an enhancement that customer's expect, like hyphenating dial strings, or sometimes it's a totally sexy homescreen widget. That part gets all the press, but all the other stuff must get done even if you decide to ship "pure android".