As /u/jschooltiger already pointed out, this is a tough question to answer and is more of an education question than a historical one, and this of course varies between texts, students, schools, regions, countries, and so on.

As a high school history teacher, WW1 is included in our curriculum (Victoria, Australia). While you said that some textbooks omit WW1, the reasons for omissions might not be too dissimilar to why textbooks leave any other point of history - there is only so much you can fit into a school curriculum, where you might only have 2 hours a week to teach kids about history.

As an example of this conundrum, our Year 7 History curriculum gives teachers the choice to cover these ancient civilisations: Greece, Rome, China, India, Egypt. That's it. Even within each section, it is extremely token, cherry picking and simplifying time periods and facts. "Ancient" Egypt merges the period so that kids come out thinking that Cleopatra was a native Egyptian princess who built the pyramids, or how "Ancient" Rome is pretty much all about Julius Caesar and the Roman Empire.

It may be better to ask why these specific civilisations were chosen, rather than why every other civilisation was not.

Ultimately, it's up the education board to decide what is taught for any field and any subject, and the best people to ask would be the district, region or state education department and the people on the committees that set out the curriculum.

Hi there! You’ve asked a question along the lines of ‘why didn’t I learn about X’. We’re happy to let this question stand, but there are a variety of reasons why you may find it hard to get a good answer to this question on /r/AskHistorians.

Firstly, school curricula and how they are taught vary strongly between different countries and even different states. Additionally, how they are taught is often influenced by teachers having to compromise on how much time they can spend on any given topic. More information on your location and level of education might be helpful to answer this question.

Secondly, we have noticed that these questions are often phrased to be about people's individual experiences but what they are really about is why a certain event is more prominent in popular narratives of history than others.

Instead of asking "Why haven't I learned about event ...", consider asking "What importance do scholars assign to event ... in the context of such and such history?" The latter question is often closer to what people actually want to know and is more likely to get a good answer from an expert. If you intend to ask the 'What importance do scholars assign to event X' question instead, let us know and we'll remove this question.

Thank you!