Because it's wise to not put too much complexity on browsers which in the end have to process and parse all inputs from front-ends. While on the rest of softwar

Because it's wise to not put too much complexity on browsers which in the end have to process and parse all inputs from front-ends. While on the rest of software development (e. g. desktop applications) you're mostly free to decide if you install untrusted code or not, the web only works if you allow untrusted code to be executed on your machine.