It's not that people don't use spoilers. They do, it's just like, for example, in the CCC OP, there is still a LOT of text to scroll over before you can get around to the important stuff. While there is some sense of organization, it doesn't feel organized enough for me. I've always had small rooms and spaces to deal with so I've come to enjoy having most of my belongings and items stored away out of sight rather than out in plain view. This reflects in the way I organize my posts even on the FCB forum thread. If I'm posting an update about what I've done concerning the challenge, the bulk of the post will be in a spoiler with a little bit of text above and below offering a softer transition.
There are people like that, which is why I try to make sure that people new to my challenge try to glance over the majority of the information before asking questions so that I can at least hope they'll know what they're doing.
Numbering serves two purposes. It can help direct you in a direction (like what to do first) or it can be used to organize information regarding people or objects (in American classrooms children are often numbered so that when the teacher is first starting the school year they'll have an easier time keeping things like textbooks, papers, and other materials organized until they learn their students' names). In the challenge-writing world, most challenge writers use numbers to tell you where to go from what point (for example, in my challenge you start at stage 1 in the core and keep going) and, in my challenge, when to stop. As for organizing arbitrary information, you can see numbering used in Minecraft to label blocks, biomes, and sub-types of those things which seems to help make coding easier.
Back to the spoiler thing: I often find it irritating when mod authors don't have the information organized in spoilers because then I have to scroll around for a bit to find what I need to know before installing the mod. And those posts can be HUGE.