Most OSes don't have render/styling systems that come close to what a browser does at their core... you can do a lot, but those features you have to often work for, or use libraries on top of the OS. And if you consider UI toolkits on top of the OS separate from the OS that's another issue.

In the end it all runs on an operating system... so it technically falls short of that of course, but building a web-based app is generally easier than raw-coding an application from the OS level.