Google outsources a lot of design and build work to external agencies. Especially for projects like this, which are sort-of side-projects and not really core Google functionality. Also, quite a lot of paid client work for YouTube is/was outsourced.
I used to work for one of these companies, and we built quite a few highish-profile sites for Google and YouTube. The quality of work done by this company varied wildly between excellent and utterly dismal. Project management interactions with the Google side were haphazard at best and chaotic and disruptive at worst. Requirements would constantly change, and the designs would be all over the place. I don't think one time we thought about accessibility. A number of the projects were death marches, and I worked on at least one huge failure, which fell over on a live TV show with millions of viewers.
TL;DR - just because it's on google.com doesn't mean it was designed or built by Google.
