I think in the case of Google, it's because they've been told they are the best developers, the top 1% of SWE's, they went through rigorous interviews, are paid a small fortune twice as much as they would get at a regular coding job, etc.

So it's dick shaking. They need to show to the world that they're better than plain HTML websites, that they have a massive schlong, that they out-chadded the vast majority of software devs. Plain html? Psht, we can invent our own language, gonna put those six years of uni to work! Wordpress? This is beneath us! It has to be a client-side rendered JS-pulled-through-GWT behemoth because on my system it's... wait it's slower, but nevermind that it's technologically ALPHA.

edit: actually looked at the source, looks like a Polymer / Web Components website. I've had to work in that once, it was dreadful compared to libraries used by real people.