I wont take extreme sides having written code across both the ends (C\C++\Assembly \embedded systems .....mobile apps .... HTML\JavaScript ), but I do see some additional reasons of why some developers can look down upon web developers.

Software development can span across a wide spectrum where :

0.0) One end of the spectrum:

Developers that write code which is just calling functions from libraries created by others and drawing fancy rectangles on the screen.

0.X) Many more types in between:....

1.0 ) The far end of the spectrum: Developers that write code which helps run mission critical systems to send pieces of metal to Mars or real-time systems for guided missiles\ nuclear reactors etc. which involves some heavy math and physics.

Barring the exception of a small set of respected folks , a large portion of web developers fall more towards the 1st side of the spectrum listed above. Sure , they may have 100's of web frameworks to learn, 50+ programming paradigms MVC\MVVM ,React \ MERN\MEAN, Scalable\ Distributed architectures, Kafka, NodeJS, VirtualDOM, MongoDb, and what not to know. But you can get the gist of why its difficult to be impressed by developers drawing fancy rectangles on screen vs those who design systems to send metal pieces to Mars .