Because it's a need of the market, using third-party plugin is no harder, maybe even easier to implement. So it's not an overhead for the site implementing it.
But rather it's an overhead for the user. you'll have to install a third-party plugin on your computer. Making it to your job now to make sure this plugin is not security vulnerable, that it is updated etc' etc'. And, it's probably closed. the web is open.
Why does that mean that we should facilitate their use of DRM? If they want to use it, they can specify a third-party plugin.