Because high level languages are where the libraries that do all of the heavy lifting exist. Libraries provide a suite of tools for absstracting away all of the complexities of creating a 'simple' web app. I think a lot of newer devs dont realise how many shoulders of giants they are standing on, and all the complexities involved in performing a simpl fetch requeust.

Sure an LLM could write it's own libraries and abstractions in a low level language, and im sure there are some assembler or c level web api wrappers, but they would be nowhere near as comprehensive or battle tested as the ones available for high level languages.

This could definitely change in the future. I think we need a coding platform that is designed for optimised LLM use, but that still allows humans to understand and write it. Kind of a markdown for code. Sort of like what OP is trying to do, but with the built in benefit of having a common shared suite of tools for interoperability.