If you were to look at all those times that PHP is cursed either for its speed or programming or something else, I am surprised these big websites are still investing in something so buggy.
Throw enough caching at a speed problem, and the speed problem effectively goes away. This is why the terms "Varnish" and "fcgi_cache" are so well associated with WordPress installs.
You are absolutely correct - caching solves the apparent speed problem to the end user, not the underlying cause of the speed problem. That said, solving the apparent speed problem for the end user is frequently enough.
As a PHP developer who has worked on several Wordpress installs as a freelancer, I can assure you companies don't invest in Wordpress because they're interested in security or speed, but so they can pay as little for the development and maintenance as possible.
WordPress is also very popular among content folks, on account of its well, content management features. Not to mention the strength of the editor, and easy extensibility. You could tap away in your terminal for a year making the new and best node.js or Go editor and by the time you were done you wouldn't be close, and WP would have moved the goalposts again. And let's say you were successful and had created a great content management system. Then you'd have to train your editors and your social media folks and they'd STILL hate it and quit and you'd have to train a new batch of editors, except, oh, you just got the opportunity to work for a company that doesn't make you spend a year re-implementing a blog from scratch and jumped on it, and now nobody knows how to get the website to let them embed a Vine? Man we should have just gone with Wordpress.
Well, I personally don't want a "special" UX in my editor. In any case just about every WP point upgrade has brought editor enhancements, so if you haven't used it recently you won't have a good basis of comparison. (e.g. 8.1 with distraction free mode – not for me personally but I've seen writers oooh over it.)
Whoa, did they use the same developers? The sites look very alike. The top navigation, squared layout, lots of white space, etc. Maybe this is the current design fad. Can't say I'm in love with it. I think all the spacing and heavy use of white space tries to mimick a print-like look, but on a screen just looks inefficient and bloaty.
I like how Ars Technica handles their layout. Its dense, not overly white, column based, and with a nice background gradient that keeps all that white at bay.
Btw, I'm a PHP developer.