For a few reasons:

- More efficient codes

- Be able to interact with anything else on the system, that can be interacted by C codes, including external programs

- More control, than using JavaScripts

A web browser designed only for advanced users, who are assumed to know what they are doing, would be a good thing to have. (And, if you do not understand something, then read the documentation; programs must have good documentation, and unfortunately many do not, and that is what makes them difficult to understand.)