Browsers have plug-in architectures. DRM systems are inherently proprietary. Leave them to implement proprietary plug-ins.
In reality though these control freak moves only "work" (for them) if everybody is forced to adopt. Another site another plugin type situation will shift a lot of people to non-drm content providers, whether on principle or maybe just plain old apathy. I'm sure browser makers could streamline this process so that it's a minimal hurdle to install a plugin but if it's optional then we have the option to avoid it and that is exactly what those goddamn morons would like to stop.