Because that is just the basic, when doing web applications, besides handling HTTP requests you need:

- workload distribution - security and authentication - role management via some form of directory services like LDAP or AD - running tasks in background in response for certain events - mapping into various kinds of databases and information sources - handling caching - have a way to manage reusable components of html/css/js + respective backend code

Yep one can make use of libraries to achieve all of that, but then most likely they don't compose in an easy way, nor make an eco-system.