The are wildly popular but they are in a niche yes. When they were not the Google we know today there were not many providing the same service or value, and potentially still isn't, so that is why they could get away with bland design. It was never bad design mind you.
Amazon is also king of providing value in their markets, and their markets are also apathetic toward flashy design. I don't need animations when I am provisioning an AWS instance nor when I am buying goods at the lowest possible price I can find.
However if I were not me, and I were shopping for luxury or boutique goods, a site that looked like Amazon would not instill me with confidence.
My point is just that the web has diverse design and UX needs and the current toolset caters for that. If someone managed to build a platform with those benefits and more and the webs market penetration then I would be on board.
I will argue still that it is necessary if you want an alternative to the web, as the alternative has to be a better value proposition for the end user not the developer.
