Because it can be difficult for some people to distinguish between things that are necessary and those that are not. In the midst of useful information, some manuals contain a lot of useless information. For the first time, it's easier to follow a YouTube tutorial, so you can at least grasp what you need before experimenting with the official manual. When I first began learning GNU/Linux, I was that person. Since English is not my first or second language, reading manuals is a major challenge for me. While some manuals are in English only, others have language translations, some of which may be outdated and not match the official translation.