Every book is a journey into a new world. In some books that world is just much further away.
It's easy to get started with a book that is set into a real world especially if it features a setting familiar to you. Even if it has some fictional elements, is set into some historical period or even in the near future it still doesn't require that much imagination to see the world. Sure there are far away places you have never been or cultures you are not familiar with in the real world too that can give you enough distance from the ordinary.
But when the book throws you lightyears away in a distant future with weird new worlds, alien races and cultures it takes a while just to get accustomed where you have ended up. It requires a lot more effort and imagination to get started with these books. It often even feels unfair that while you are just trying to figure out where you are you should already get yourself familiar with the characters f the book and their doings (bonus points for those books that wont even let you this easy but throw in some time shenanigans too).
Despite the additional effort such books require in the end they usually end up being the most interesting and thought provoking books.