You don't even need a fancy "send html fragments over the wire" approach to create a better user and developer experience. Just sending full pages, server side rendered, like Hacker News and Wikipedia do is fine: Going from the HN homepage to this topic we are on: 36 KB in 5 requests. Going from the Wikipedia Homepage to an article: 824 KB in 25 requests Going from the AirBnB homepage to an apartment listing: 11.4 MB…
Both AirBnB and Reddit are incentived to collect a massive amount of data on their users and to maximize the experience in any way possible. They need the data to understand user behavior and feed it back into the recommendation algorithms. For both hospitality and social media, users look for images/video content since it is much more powerful than text for most people. All of their competitors have equally bad performance for similar reasons, so they don't lose a competitive edge.
Hacker news specifically goes for a retro look with an interface that has not changed significantly in decades. Wikipedia simply doesn't have the money or engineering capacity to support a more robust platform like airbnb or reddit.