It is a very complex set of tasks and to top it all off SEO changes dramatically all the time, so the fundamental best practices are often very very similar from year to year but there's always nuance, new additions, new opportunities, new ways that the search engines are changing up what the results look like, how you ...
The answer is simple. Learning SEO is hard because there is a lot of information to read about search engines and how the optimization process works, and this can be overwhelming at first.