It is not necessarily about scaling the number of requests, but scaling for developers & security. You will pretty soon hit a point where you need smarter cookies access, CSRF protection, multipart/form-encoded POST requests etc. Sure, you could implement everything yourself, but this is stuff those "frameworks" already solved.

I think it is also worth to mention there are plenty of "slim/light" frameworks that do very little (i.e. take care of threading, provide html request/response as writable streams, have the http headers parsed for you into a Map-like data structure etc.). Those lightweight frameworks exist i.e. express without middleware (nodeJS), Nancy (.NET), Flask (Python) etc. The "full-blown" counterparts would be ASP.NET or Django which bring far more to the table than you might need.