1. Because for example, there’s the left sidebar widget area. It wouldn’t make sense for it to appear any other time unless you specifically chose the left sidebar layout for a page. This is a logistics thing. Is it fair to assume you’re relatively new to WordPress? If so, please understand that this is how all themes handle such features and may make more sense the more time you spend with it.
2. Use:
#pages-2 {
-moz-column-count: 4;
-webkit-column-count: 4;
column-count: 4;
width: 100%;
}