Why Is It Hard to Write a Good Cover Letter?

hello i am Adrian Byrne i am a web designer i want to ask you something really important.If I am looking for a flexible job,One of my friend suggested it's really important to have a cover letter that shows why I am a good fit for that type of work.But Writing a cover letter seems really difficult for me. Why do so many people struggle with it, and how can a cover letter writing service ireland make this process easier for someone like me?