Because some people think that Browsers should be the ultimate app platform, so hijacking your inputs or preventing proper zoom makes sense to them even though it's utterly user hostile.
Which has led us to regress in terms of building GUI desktop apps. Remember the 2000s and 90s how you could make a somewhat native UI in Visual Basic 6 and Delphi, now you got to use an entire browser to get there.
Java didn't only run on one OS but for whatever reason Oracle seems to find no value in making its UI stack nicer and modern. It seems only Microsoft's C# stack is working on this. There's also Qt, which is not a single-OS solution, but its C++.
Well, there is JavaFX, which is really nice for modern java desktop UI's. However, Swing can be taken really really far - just look at IntelliJ and friends (most people don't even realize they're Swing UI's).
With that said - most modern applications are webapps for good reasons. Making native apps sucks for a lot of reasons - including all the random OS-specific behavior you have to work around, specific versions of native OS libraries, etc.
Building for the web browser means, without any extra effort, you app works on all operating systems, and it works exactly the same. That's a pretty good sell to anyone trying to make a modern application that's mostly just a front-end UI for an API...
I think it's got a certain "cool" factor. For some types of content in some cases it might also make sense, almost like a slideshow or paginated document, allowing each jump have its own self-contained focus.
For example this page has action buttons and community links with a video on the first page, a written blurb about system configuration on the second page, screenshots/visual showcase on the third page, etc.
Personally I'm on the fence. I think it probably more often than not tends to be annoying, like most overrides of default conventions, and feels more like you're being force-fed marketing instead of reading information. This specific page would probably work just as well if they kept the sections, dividers, and even the animations, but just let you scroll naturally.
But then, maybe it gets them more downloads or something.