Nobody likes to download and install stuff. Just imagine the state of the web if you had to install a app for every website you visit ? Oh...you need to add iOS and Android to the list of platforms. That makes it 5 platforms. 5x the work.
I don't want to ask everyone to install a native app, I don't want to use several different native app UI libraries, etc. I'm one of those people who thinks that the web browser as an app platform is a pretty good idea.
Make a native app for every independent platform?
Windows, Linux, OSX - there just aren't that many platforms these days, it's perfectly do-able. If your core logic is in ANSI C (or equivalent) and you just need a GUI for each platform, even easier.
It's still 3 more platforms, which means 3 more developers, which leads to more $$$. Not everyone is VC backed with multi-millions.
But one of these "offline" apps has to work in Firefox, IE and Safari, so really, it's no different
As long as you stick to the APIs implemented in each browser, your code can be exactly identical. Realistically, you'll probably need to add some polyfills and edge case handling for IE, but it's much less than developing for different full-on platforms.
With the major exception that all evergreen browsers (most browsers in wide use today) quickly officially support new web standards, but Windows, OS X, and Linux are unlikely to ever officially support each others' native toolkits.
Don't forget iOS, Android and Windows Phone.
Because not every news website and blog needs an app. I think everybody is tired of seeing the "Install our app" banners for every website you visit.
News websites and blogs don't need to work offline. Even if they did, "save page as" or an RSS reader was the way to go.