I think that for a lot of people (including me), it's procrastination and fear that makes it harder than it is..
According to a recent survey, 81 percent of Americans feel they have a book in them -- and that they should write it
This means that a little over 4/5 of the U.S population feels as if they have a story worth telling. Now, assuming all 81% actually get into the stages of writing a book, most of them don't go the distance because of the difficulty..
Why?
1. They don't think they can write.
People have an 'I'm not a writer' mentality. This basically means that they feel as if they would not be able to write something that other people can sell. They believe that, since they don't write much, they are destined to never write a book.
If you don't think you can write, it will be extremely difficult for you to power through. Thinking too much about your "skill level" and "natural ability" can make it hard for the writer. Being too harsh on yourself will make the writing process a nightmare.
New's flash: Every person can write. You, the person reading this, can write. I can write. My best friend can write. It's just up to you to see whether or not you can use the techniques developed by writing. A lot of the time this enhances the writing, but you don't HAVE to do it. There is no 'right' way to write, even grammar is ever-changing in the English language and isn't even taught in many English classes in the U.S.
2. A break can make or break the difficulty.
For many people, taking a break from their novel helps them regain interest in it. For me (and I'm sure other people), it may be difficult to get into the groove as you did before. This can make writing it confusing, as you may forget specific details or just can't pick something else for a plot.
3. It's difficult to stick to one idea
Similarly, what makes writing a novel so difficult is sticking with one overall idea. Sure, you can change minor things, but making a complete 360 in a novel's plot or genre (for the most part) will be difficult to explain to readers. There is this thing where writers will have an amazing idea, write a few chapters, and then lose complete interest in the story that they're telling. It's very difficult to write something if your heart isn't in it. It'll feel more like a chore than something you want to do.
There are other reasons it may be difficult, too. These are just some of the most common ones I could think of that may explain why you feel like writing is hard.