The reason is that you're reading TeX, not LaTeX. The latter has abstracted away the fundamental building blocks so few people know how an hbox is set anymore. So, an hbox is a box where the content is in horizontal mode. Between the things is glue. Glue can stretch and shrink. Depending on how you have set your tolerance and penalties, there's a maximum percentage of stretch allowed. If the glue stretches more, it becomes bad, this is called badness and can effectively be up to 10000 bad. So why not just put more things into the box? Well, (La)TeX probably tried to do that, but came up with worse badness. TeX always chooses the least bad option on a paragraph level. In practice, the usual suspect is often that you have something else that can't fit the last part of a line, like a really long word. If you can look at it and manually hyphenate it, things might be better.