Native applications have orders of magnitude more access to your system than a website has.

This isn't a loaded question, but how come people (especially the people here) don't know that?

the real question is: Why do people write websites to behave more and more like native apps? In order to prevent a client-side install?

For applications that want special access to my machine, there SHOULD be a barrier to entry or inconvenience like a client-side installation.