These are some of the reasons why book writing is difficult, according to Book Writing Services USA.

It isn’t easy to write a book:

Many people avoid writing a book because they find it difficult. The vast majority of people are unable to do so. Forcing oneself to sit down, brainstorm, write, edit, rewrite, edit, trim, add, rewrite, workshop, rewrite, and rework until you have somewhere between 50,000 and 100,000 words is exhausting labor. It makes you feel good about yourself to be one of the few who can do it—a necessary quality in a true badass.

Editing is Excruciating:

All of your hard work and time spent creating a scene could be for naught if it doesn’t perfectly fit into your story. It doesn’t matter if it’s your favorite scene: if it’s inappropriate for your book, it has to go. It can be agonizing to delete your writing, especially words you’ve worked so hard to create. But you do it anyway because, deep down, you’re a tough cookie and you know it’ll help your book.

It is impossible to know when you are finished:

Is your book’s first chapter strong enough? Do you devote enough time to demonstrating rather than telling? Should your main character walk or hike in this one scenario? You’re on draught #17, and after rereading it, you think an 18th draught is in order.
“Finished” is a relative term when it comes to the end of a baseball game or a Broadway performance, but it isn’t when it comes to writing. After all, there’s no way to tell if your manuscript is error-free.