Because we want the information to be free. If the web server is serving a document, you can do all sorts of stuff with it - you can index it, you can transform it, you can save it for later.

If the web server is serving a DRM-ed program, that loads the human-viewable data over non-standard interfaces, all that breaks. Only humans in front of the web browser will be able to see the data.

Or sufficiently dedicated people to run a headless browser to run the Javascript and re-build the content and work on the rebuilt DOM. But also we now need NoScript, JS blockers, 3rd party blockers, and the publishers invest in anti-adblocks. It's a neverending arms escalation between those who want to restrictively publish information, and those that want the information without restrictions. So all this JS-based workaround to try to DRM things only brings more work for everybody involved, with minimal results.

NOTE: when I say DRM, I actually mean Digital Policy Enforcement - the publishers want to maintain their policies around access to their information (e.g. you cannot see this article without seeing this ad) using digital means. But DRM has a nicer twist to it - the uninformed may mistake the R for My Rights.