I think you're attributing to insecurity something which could be equally attributed to the sort of hubris which engineers are infamous for. To me, my problem domain is rich and varied, but your problem domain is mostly trivial. I understand HTML, stylesheets, server-side scripting, and client-side scripting at an intellectual level. Ergo, all problems in the web programming arena must be easily comprehended.
To a web programmer, however, the web development field is full of interesting, challenging, or frustrating issues such as incomplete or inconsistent standards implementation between browsers, supporting legacy browsers, applying solid visual design principles while also applying solid software engineering principles, dealing with a single-threaded execution environment for fundamentally parallel operations, falling back gracefully to older technologies when newer ones aren't installed on the client side...and so on.
So the systems programmer who looks at web programming and says "pft, trivial. A solved problem" is not necessarily exposing his own insecurity, merely the blinders that he is wearing to the depth and breadth of problems which do not interest him.
Sure, there are many potential reasons. Hubris could also be rooted in insecurity for that matter. My point has very little to do with the underlying reasons though.
The point is all programming has big challenges, and the list you cite for web programming doesn't really do it justice. Web programming encompasses all the toughest issues of network and scalable computing, a set of theoretically difficult problems as deep as anything in systems programming.
To a web programmer, however, the web development field is full of interesting, challenging, or frustrating issues such as incomplete or inconsistent standards implementation between browsers, supporting legacy browsers, applying solid visual design principles while also applying solid software engineering principles, dealing with a single-threaded execution environment for fundamentally parallel operations, falling back gracefully to older technologies when newer ones aren't installed on the client side...and so on.
So the systems programmer who looks at web programming and says "pft, trivial. A solved problem" is not necessarily exposing his own insecurity, merely the blinders that he is wearing to the depth and breadth of problems which do not interest him.