If the ICA explicitly asks for a cover letter, all applicants will start sending many cover letters, most of which hold little to no value. Similarly, asking for additional documents will only result in applicants piling up as much paperwork as they can to make their cases stand out as much as possible.
Also, it will increase the time ICA takes to process an application as there will be extra paperwork for them. On the other hand, submitting your application with a cover letter and well-planned documents will give your application an edge as long as they contribute to your unique case.