First, Google didn't design anything. They were looking for partners for their OS and these partners used their existing design. The first HTCs were almost identical between their Android and Windows Mobile versions.
Second, there are reasons why the embedded systems do not have them. Apart from increased complexity (bad for the designer and manufacturer), increased energy consumption (bad for consumer, and being a competitive disadvantage too), very few of both, embedded devices manufacturers and customers, even count on using other software than the one supplied. From the point of view, there every cent of saved costs on the device makes millions in margin, that would be wasted money.
Microsoft basically strongarmed the PC vendors - they either did what Microsoft said, or didn't ship Windows with their wares. Windows, which was the only game in the town, if you wanted to sell PCs.
Google didn't have such luxury when they started with Android. They needed to be everyone possible to be with them onboard and the "lets throw out everything you have and design new hardware from scratch" doesn't make for a good start.
In mobile, Microsoft also reused Qualcomm's reference design. But contrary to Google, they used ONLY Qualcomm's design, that's why their system looks united. All the WP phones are basically the same board.
However, that does not mean you will get an open device now. When was the hardware openness so important, that it played a major role in purchase decision at statistically significant rate?
For 99,999% of people, it doesn't. They want an appliance that works out of the box, without bothering with alternate firmwares. So that's what they are getting.
There's no motivation to put UEFI and PCI into the hardware, just like there wasn't 10-15-20 years ago, when the first designs were made.