Why does Google need OEM's for the Nexus?

If Google can design and manufacture products like the Chromebook Pixel, why does it need OEM's to manufacture their Nexus line? I feel like they could have a more consistent/appealing line-up if the design was done in house.

The argument I've seen in the past is that the OEM's would not be happy if Google competed with them directly since they would have some kind of advantage by producing the software. I don't understand this theory though. The Nexus line runs stock android. What advantage could Google possibly have? Even if Google were to compete with the OEM's it's not like they're all going to drop Android for Windows at this point.

Furthermore, Google should really be hedging for a future where Samsung drops Android for Tizen.