The replies to Maciej's tweet are interesting to read. They fall roughly into two camps:
Older/not front-end developers: because the web is shit!
Current front-end developers: because shit is hard!
As is often the case, both camps are correct! The web is a shitshow of wheel reinvention and bad APIs. It's also a blizzard of innovation.
Expectations for what a website should be able to do have evolved enormously. Users expect snappy, desktop-like responsiveness and rich presentation in web apps. They also expect those same web apps to work equally well on mobile devices. And they expect these apps to load basically instantly.