To address your first question: this has to do with scope insensitivity, hyperbolic discounting, and other related biases. To put it bluntly, most humans are actually pretty bad at maximizing expected utility. For example, when I first head about x-risk, my thought process was definitely not "humanity might be wiped out - that's IMPORTANT. I need to devote energy to this." It was more along the lines of "huh; That's interesting. Tragic, even. Oh well; moving on..."

Basically, we don't care much about what happens in the distant future, especially if it isn't guaranteed to happen. We also don't care much more about humanity than we do about ourselves plus our close ones. Plus we don't really care about things that don't feel immediate. And so on. Then end result is that most people's immediate problems are more important to them then x-risk, even if the latter might be by far the more essential according to utilitarian ethics.