Because you would need to reproduce an explicit Google copyright statement which states that you don't have the right to copy it ("All rights reserved.") in order to do it fully.
That presumably gives Google the legal ammunition it needs to sue you if you do it.
I'm no lawyer, but my take on it is that by reproducing this particular value for the validation header, you are stating that you are the Chrome browser. It's likely that this has been implemented in such a way that other browsers could use it too if they so choose; the expected contents of the copyright header can then change depending on what you have in the validation header.
To me, it seems likely that the spec is for a legally defensible User-Agent header.
Companies like SEGA have tried doing stuff like that in the past, and lost.