That doesn't mean they're easy. I mean that there's a pretty well accepted baseline of things to cover in those courses. Less so with linear algebra, as some math purists throw tantrums about determinants and argue over how much of the course should be applied vs pure. But regardless of where you stand on that, there's a textbook or two out there that has been widely taught and vetted.

Higher level courses often require far more prep time because you don't have a bunch of existing syllabi and expectations you can draw from. There's no widely aggreed upon standard on what should be taught in a distributed systems course, for example, and it's not like you can just grab a textbook that everyone uses and pick chapters from it to cover. There will inevitably be some textbooks for it, but they're usually awful and out of date, and relying on them will make your students miserable.

A great example of an easier course that's not "basic" would be introduction to programming. There's really no standard for what to cover, and teaching it in a way that makes its topics valuable and interesting to the students requires a lot of thought and time. And of course these classes are often taught by overworked adjuncts, TAs, or profs doing 4/4 teaching load, so they just find a textbook and make students do rote exercises with completely inappropriate methods (having students mix loop practice with GUI development by having them incorporate them into boilerplate heavy Java/C# GUIs for example)