The issue is that "important parts" clarification, because what's "important" varies widely from person to person, table to table..
Some people will read over these rules, get the surface level of the setting, and basically just do their own thing using the mechanics: making up characters, how each nation works, the map, etc. These sorts of games will probably be totally alien from how Roshar and the Cosmere actually is, and that's a perfectly great way to play, and lots of fun. (That's how I handle 90% of the games I've played with baked-in settings, and I and my players have a great time.) If that's what you're referring to, those people can play literally anything and it won't matter how much or little pre-existing setting there is.
But for people who really care about playing games within as canonically accurate a setting as possible, whether they are initially familiar with the setting or not, there's a LOT to learn in the Stormlight setting alone, let alone the entire Cosmere. I wouldn't say it's as big as Star Wars, but trying to play a game that sticks as close to canon as possible makes me think of an awful experience I had playing Star Wars with some heavy Star Wars lore nerds who would not do anything until they figured out everything that already existed about every area, every character, etc. I felt like I had to take a college-level course on Star Wars to keep up with those guys.
Obviously most groups aren't going to fall into either of those two extremes. In fact, most groups are going to run closer to the former than the latter. But not every TTRPG player is able to get the most important parts of a setting at a glance, and not all are interested in a setting where they may feel they need to learn a lot, even if they don't have to. (And I totally understand people in that situation—adult life is busy, and it's no fun sitting at a table and feeling you're missing out on things because everyone else has read thousand-page books that you haven't.)
